Adverse effect of topical methylsalicylate ointment on warfarin anticoagulation: an unrecognized potential hazard

Risks and benefits of salicylates in food: a narrative review

Salicylates are generally present in plants as part of their defense system against pathogens and environmental stress. Major dietary sources of salicylates were found in spices and herbs, such as curry and paprika (hot powder). Several studies suggest that these natural salicylates offer health benefits in the human body, such as antidiabetic, anticancer, antiviral, and anti-inflammatory properties. However, despite their advantages, salicylates can be harmful to people with allergies, and high doses of salicylates may cause respiratory alkalosis and gastrointestinal bleeding. Additionally, salicylates can interact with certain drugs, such as nonsteroidal anti-inflammatory drugs and warfarin. This narrative review aimed to consolidate recent information on the content of salicylates in food based on the literature, while also highlighting the benefits and risks associated with salicylate consumption in humans. Based on the literature review and analysis of results, it can be concluded that the dietary intake of salicylates in vegetarians can be relatively high, resulting in concentrations of salicylic acid in the blood and urine that are comparable to those observed in patients taking a low dose of aspirin (75 mg). This suggests that a diet rich in salicylates may have potential benefits in preventing and treating some diseases that require low doses of aspirin.

Warfarin and food, herbal or dietary supplement interactions: A systematic review

AIMS

To present an updated overview on the safety of concurrent use of food, herbal or dietary supplement and warfarin.

METHODS

A systematic literature review was performed on 5 databases from inception up to 31 December 2019. These interactions were classified depending on the likelihood of interaction and supporting evidences.

RESULTS

A total of 149 articles describing 78 herbs, food or dietary supplements were reported to interact with warfarin. These reports described potentiation with 45 (57.7%) herbs, food or dietary supplements while 23 (29.5%) reported inhibition and 10 (12.8%) reported limited impact on warfarin pharmacokinetics and pharmacodynamics. Twenty unique herb and dietary supplements also reported to result in minor bleeding events, such as purpura and gum bleeding as well as major events such as intracranial bleeding that led to death.

CONCLUSION

While most food, herbs and supplements can be safely taken in moderation, healthcare professionals should be aware of the increased risk of bleeding when taking several food and herbs. These include Chinese wolfberry, chamomile tea, cannabis, cranberry, chitosan, green tea, Ginkgo biloba, ginger, spinach, St. John's Wort, sushi and smoking tobacco. Patients should be counselled to continue to seek advice from their healthcare professionals when starting any new herbs, food or supplement.

Bismuth Subsalicylate Coagulopathy in a Patient with Chronic Liver Disease

Bismuth subsalicylate (BSS) is the active ingredient in over-the-counter antacid and antidiarrheal medications. Coagulopathy in the setting of acetylsalicylic acid toxicity is well documented but not in setting of bismuth subsalicylate overuse. We present a case report of coagulopathy from BSS poisoning in a patient with underlying cirrhosis. The patient's high prothrombin time suggests inhibition of vitamin K-dependent coagulation factors. The patient had decreased factor V activity, which is responsible for converting prothrombin to thrombin. Patients with cirrhosis often have hypoprothrombinemia which may be exacerbated by salicylate-induced coagulopathy. Given the widespread use of BSS products, physicians should recognize coagulopathy as a possible manifestation of toxicity especially in patients with underlying liver disease.

How Evidence-Based is our Practice in a Hong Kong Emergency Department?

Objective

To evaluate how evidence-based our daily practice was.

Design

Retrospective study.

Setting

Emergency department of a public district hospital.

Patients and Methods

Between 1st August 2000 to 7th August 2000, 91 patients' records were chosen at random. A chief diagnosis was assigned for each patient. Corresponding treatments were reviewed by searching relevant randomised controlled trials (RCTs), systematic reviews and meta-analyses. Each patient had only one chief diagnosis but could have multiple interventions for that diagnosis.

Results

Out of 91 records, 14 were discarded. All of them had not been given any intervention and 11 required admission. For the remaining 77 records, there were 38 subjects in medical, paediatric, or gynaecological specialties and 39 in surgical or orthopaedic specialties. Intervention(s) given for each subject were then searched electronically through our hospital Knowledge Gateway and the results were expressed as either EBM-positive or EBM-negative. “EBM-positive” interventions denoted a support by RCTs. “EBM-negative” interventions denoted an absence of any supportive RCTs. Each patient might have EBM-positive and/or EBM-negative interventions together if that patient received more than one treatment. There were 52 patients (52/77 = 68%) who had one of their interventions being RCT-supported. The majority were patients with (1) antipyretic use of paracetamol in upper respiratory tract infection, or (2) control of pain by nonsteroidal anti-inflammatory drug, dologesic and paracetamol. There were 25 patients (25/77 = 32%) who did not receive any RCT-supported interventions. Concurrently 53 patients out of 77 (69%) received EBM-negative interventions. The majority were patients with (1) the use of antibiotics, antitussives and antihistamines in upper respiratory tract infection, (2) antispasmodics in gastroenteritis or patients with nonspecific abdominal pain, and (3) the use of analgesic balm in minor orthopaedic complaints.

Conclusion

Sixty-eight percent of patients had EBM-positive interventions. Thirty-two percent of patients did not receive any EBM-positive intervention. It was quite encouraging as compared to studies in other specialties with similar design. Concurrently 69% of patients had also been given EBM-negative interventions. There were areas for improvement if we were to implement EBM practice in the emergency department.

Topical NSAID formulations

OBJECTIVE

This article reviews topical nonsteroidal anti-inflammatory drug (NSAID) formulations available in the United States, including advantages and disadvantages, therapeutic usefulness, adverse effects, and formulation considerations.

RESULTS

In the United States, several topical NSAID products are approved to treat painful conditions including diclofenac sodium 1% gel (Voltaren Gel®; Endo Pharmaceuticals), diclofenac sodium topical solution 1.5% w/w in 45.5% dimethyl sulfoxide (PENNSAID®; Mallinckrodt, Inc.), and diclofenac epolamine 1.3% (Flector Patch®; Alpharma Pharmaceuticals LLC, a subsidiary of Pfizer, Inc.). Recent studies suggest topical diclofenac preparations are effective for osteoarthritis pain and suggest the efficacy of topical formulations is similar to that achieved with oral NSAID formulations. All NSAID formulations contain the same boxed warnings regarding cardiovascular and renal toxicity; however, topical NSAIDs are proposed to have a more favorable safety profile than oral NSAIDs due to the low serum concentrations achieved with topical NSAID formulations.

CONCLUSIONS

Topical NSAIDs have been shown to be beneficial from both a therapeutics and adverse effect perspective, and are increasingly recommended in treatment guidelines.

Inadvertent exaggerated anticoagulation following use of bismuth subsalicylate in an enterally fed patient receiving warfarin therapy

We report a case of an inadvertent increase in the international normalized ratio (INR) after the addition of bismuth subsalicylate for the treatment of diarrhea in an enterally fed patient receiving warfarin therapy. A 56-year-old Caucasian female presented to the trauma intensive care unit (ICU) with multiple lower extremity fractures. Warfarin was initiated for deep vein thrombosis prophylaxis due to the patient's inability to ambulate. The target INR was 2-3. Continuous intragastric enteral feeding was withheld 1 hour before and 1 hour after intragastric administration of warfarin. Bismuth subsalicylate 30 mL every 4 hours was prescribed for diarrhea. Within 3 days after starting bismuth subsalicylate therapy, the patient's INR increased from 2.56 to 3.54 and minor bleeding was noted from the patient's tracheostomy site. No significant change in warfarin dosage, variability in vitamin K intake, or medications that potentially alter warfarin metabolism were present during the unexpected rise in INR. When the bismuth subsalicylate was discontinued, the patient's INR stabilized into the target range on the same warfarin dose given at the time of the supratherapeutic INR. Salicylate displaces warfarin from plasma protein binding sites and may result in a significant increase in INR secondary to redistribution of warfarin to the free active form. Evaluation of this case report using the Drug Interaction Probability Scale and Naranjo Adverse Drug Reaction Probability Scale yielded scores consistent with a probable adverse drug interaction. Bismuth subsalicylate exaggerates warfarin's anticoagulant response and its concurrent use during warfarin therapy should be avoided.

A review of pharmacotherapy for chronic low back pain with considerations for sports medicine

Up to 30% of athletes experience low back pain (LBP) depending on sport type, sex, training intensity and frequency, and technique. United States clinical guidelines define back pain as chronic if it persists for ≥ 12 weeks, and subacute if it persists 4 to < 12 weeks. Certain sports injuries are likely to lead to chronic pain. Persistent or chronic symptoms are frequently associated with degenerative lumbar disc disease or spondylolytic stress lesions. Exercise therapy is widely used and is the most conservative form of treatment for chronic LBP (cLBP). Pharmacotherapies for cLBP include acetaminophen, nonsteroidal anti-inflammatory drugs, and opioids. Acetaminophen is a well-tolerated first-line pharmacotherapy, but high-dose, long-term use is associated with hepatic toxicity. Nonsteroidal anti-inflammatory drugs can be an effective second-line option if acetaminophen proves inadequate but they have well-known risks of gastrointestinal, cardiovascular, and other systemic adverse effects that increase with patient age, dose amount, and duration of use. The serotonin-norepinephrine reuptake inhibitor, duloxetine, has demonstrated modest efficacy and is associated with systematic adverse events, including serotonin syndrome, which can be dose related or result from interaction with other analgesics. Opioids may be an effective choice for moderate to severe pain but also have significant risks of adverse events and carry a substantial risk of addiction and abuse. Because the course of cLBP may be protracted, patients may require treatment over years or decades, and it is critical that the risk/benefit profiles of pharmacotherapies are closely evaluated to ensure that short- and long-term treatments are optimized for each patient. This article reviews the clinical evidence and the guideline recommendations for pharmacotherapy of cLBP.

Bleeding risks of herbal, homeopathic, and dietary supplements: a hidden nightmare for plastic surgeons?

The utilization of complementary and alternative medicine has increased tremendously in the last two decades. Herbal products, homeopathic medicines, and dietary supplements are extremely popular and are available without a prescription (which likely contributes to their popularity). Despite their "natural" characteristics, these remedies have the potential to cause bleeding in patients who undergo surgery. The high use of these supplements among cosmetic surgery patients, coupled with increasing reports of hematomas associated with herbal and homeopathic medicines, prompted the authors to conduct a comprehensive review focused on bleeding risks of such products in an effort to raise awareness among plastic surgeons. This review focuses on 19 herbs, three herbal formulas, two herbal teas, and several other supplements that can cause bleeding perioperatively and postoperatively. In addition to being aware of such adverse effects, plastic surgeons must adequately screen all patients and educate them on the possible dangers associated with these treatments.

Analgesics and anti-inflammatory medications in sports: use and abuse

Both acute and overuse musculoskeletal injuries are common in adolescent athletes. Pharmacologic agents including nonsteroidal anti-inflammatory drugs, acetaminophen, and topical over-the-counter agents have been shown to be effective in controlling pain, but data regarding their efficacy in expediting healing and time to recovery continue to be debated. Studies indicate that adolescents consume analgesic agents on their own and may be unaware of their potential toxicities. Data also indicate that adolescent athletes use medications in hopes of alleviating pain and allowing continuation of sports without adequate time for healing. This article reviews the mechanisms, toxicity, drug interactions, efficacy, and abuse potential of commonly used analgesic and anti-inflammatory drugs.

Adverse effects of topical nonsteroidal antiinflammatory drugs in older adults with osteoarthritis: a systematic literature review

OBJECTIVE

To systematically review the literature on reported adverse effects (AE) associated with use of topical nonsteroidal antiinflammatory drugs (NSAID) in older adults with osteoarthritis (OA).

METHODS

A systematic search of Medline (1950 to November 2009), Scopus, Embase, Web of Science, Cochrane databases, Dissertation and American College of Rheumatology meeting abstracts was performed to identify original randomized controlled trials, case reports, observational studies, editorials, or dissertations reporting AE from topical NSAID in older adults with OA. Information was sought on study and participant characteristics, detailed recording of application site, and systemic AE as well as withdrawals due to AE.

RESULTS

The initial search yielded 953 articles of which 19 met eligibility criteria. Subjects receiving topical NSAID reported up to 39.3% application site AE, and up to 17.5% systemic AE. Five cases of warfarin potentiation with topical agents were reported, 1 resulting in gastrointestinal bleeding. In formal trials, the withdrawal rate from AE ranged from 0 to 21% in the topical agents, 0 to 25% in the oral NSAID, and 0 to 16% in the placebo group.

CONCLUSION

Although topical NSAID are safer than oral NSAID (fewer severe gastrointestinal AE), a substantial proportion of older adults report systemic AE with topical agents. The withdrawal rate due to AE with topical agents is comparable to that of oral NSAID. Given the safety profile and withdrawal rates described in this study, further data are needed to determine the incremental benefits of topical NSAID compared to other treatment modalities in older adults with OA.

Life-threatening retroperitoneal bleeding due to warfarin-drug interactions

Warfarin-drug interactioions, which can result in life-threatening bleeding, are preventable. A 53-year-old man was admitted to hospital with exacerbation of chronic obstructive airways disease, cor pulmonale, pneumonia, deep vein thrombosis and acute pulmonary embolism. His pulmonary thromboemobolism was initially treated by low-molecular-weight-heparin and heparin. After a loading dose of 5 mg for 2 days, warfarin was given in a daily dose of 2 mg. On the fifth day of warfarin therapy, the last dose of Enoxaparin was given in the morning. He had a fall in the bathroom with blunt injury to the right flank. He complained of right thigh numbness and increasing pain and swelling over his right flank and abdomen. A tender mass was noted over the right flank. His Hb level dropped to 9.7 g/dl. His INR increased from 2.46 to 3.49-3.71 one day later. On further questioning, he admitted self applications of 20 g of Analgesic balm (50% methyl salicylate) over his right calf for 3 days. CT scan showed a large right retroperitoneal haematoma and right iliacus intramuscular haematoma. Warfarin was withheld. He was given fresh frozen plasma, packed cells and vitamin K(1). Inferior vena cava filter was inserted. The haematomas were resolving. He was subsequently discharged to convalescence hospital for continuation of anticoagulant therapy and close monitoring. Significant usage of topical methyl salicylate ointment can potentiate the anticoagulant effect of warfarin. The over-anticoagulation and the presence of platelet dysfunction increase the risk of severe bleeding, which can be provoked by trauma.

Topical therapy for osteoarthritis: clinical and pharmacologic perspectives

Nonsteroidal anti-inflammatory drugs (NSAIDs) have shown efficacy in patients with osteoarthritis (OA) pain but are also associated with a dose-dependent risk of gastrointestinal, cardiovascular, hematologic, hepatic, and renal adverse events (AEs). Topical NSAIDs were developed to provide analgesia similar to their oral counterparts with less systemic exposure and fewer serious AEs. Topical NSAIDs have long been available in Europe for the management of OA, and guidelines of the European League Against Rheumatism and the Osteoarthritis Research Society International specify that topical NSAIDs are preferred over oral NSAIDs for patients with knee or hand OA of mild-to-moderate severity, few affected joints, and/or a history of sensitivity to oral NSAIDs. In contrast, the guidelines of the American Pain Society and American College of Rheumatology have in the past recommended topical methyl salicylate and topical capsaicin, but not topical NSAIDs. This reflects the fact that the American guidelines were written several years before the first topical NSAID was approved for use in the United States. Neither salicylates nor capsaicin have shown significant efficacy in the treatment of OA. In October 2007, diclofenac sodium 1% gel (Voltaren Gel) became the first topical NSAID for OA therapy approved in the United States following a long history of use internationally. Topical diclofenac sodium 1% gel delivers effective diclofenac concentrations in the affected joint with limited systemic exposure. Clinical trial data suggest that diclofenac sodium 1% gel provides clinically meaningful analgesia in OA patients with a low incidence of systemic AEs. This review discusses the pharmacology, clinical efficacy, and safety profiles of diclofenac sodium 1% gel, salicylates, and capsaicin for the management of hand and knee OA.

Review of the pharmaceutical properties and clinical effects of the topical NSAID formulation, diclofenac epolamine

BACKGROUND

Topical formulations of non-steroidal anti-inflammatory drugs (NSAIDs), in particular diclofenac (DI), have become popular for treating various acute and chronic painful inflammatory conditions.

OBJECTIVE

To perform a literature review of (1) the use of topical NSAIDs; (2) the pharmaceutical, pharmacokinetic and pharmacodynamic properties of a medicated plaster (patch) containing diclofenac epolamine (DI-EP, Flector Tissugel, Flector patch) compared with other formulations of topical NSAIDs; and (3) evaluation of the clinical findings from studies with this novel DI-EP patch.

OUTCOMES

(1) Pharmacokinetic studies involved determination of DI from DI-EP and separately epolamine (EP) and the epoxide metabolite (N-oxide-EP) in laboratory animals and humans; the latter being the major metabolite in humans. About 2% of DI is absorbed by the skin in humans and is excreted in the urine. Maximum plasma concentrations of 17.4 ng/mL DI are reached at 5.4 hours (approximate steady state conditions); the plasma elimination half-time (t(1/2)) being 26.4 hours. Low systemic levels of DI and EP are produced from DI-EP. Pronounced accumulation of DI occurs in the muscle layers and in synovial fluids of arthritic patients; (2) No significant toxicity occurs from EP nor N-oxide-EP, while that of oral DI-EP was similar to that from DI; and (3) In acute musculoskeletal conditions (sprains, tendonitis and sports injuries) and osteoarthritis DI-EP patches control pain and signs of joint or physical injury compared with placebo controls by 3-5 days with almost complete pain relief at 14 days. DI-EP was shown to have equivalent therapeutic effect to another DI diethylammonium gel formulation (Voltaren Emulgel). There were no reports of serious adverse events in the gastro-intestinal (GI) tract, kidneys or liver from DI-EP. Mild GI symptoms and skin reactions occur in 2 and 10% of patients, respectively.

CONCLUSIONS

The patch delivery of DI in DI-EP affords controlled delivery of the active drug in contrast to that from application of gels or ointments of NSAIDs.

Potential adverse and allergic reactions from complementary and alternative medicine and dietary supplements

: Complementary and alternative medicine and dietary supplements are often used by patients. A detailed examination of each preparation used by four patients was carried out. Seven such preparations with the potential to cause bleeding, cardiovascular and central nervous system side effects, and allergic food reactions are described. They were taken by both Asian and Caucasian patients, were purchased locally, and were used for allergic and nonallergic disorders. Inquiry into their use is important to prevent potential adverse and allergic reactions. There should be a higher standard of regulation for such products.

Potentiation of warfarin anticoagulation associated with topical methyl salicylate

OBJECTIVE

To report a case of international normalized ratio (INR) elevation resulting from the administration of topical methyl salicylate in a patient whose INR was previously stable while she received warfarin anticoagulation.

CASE SUMMARY

A 22-year-old white woman presented with an INR of 12.2 after applying a topical pain-relieving gel to her knees daily for eight days. The potentiation of the warfarin anticoagulation was attributed to the low-dose methyl salicylate contained in the product.

DISCUSSION

Methyl salicylate is systemically absorbed through the skin in measurable amounts, and may increase warfarin action by affecting vitamin K metabolism or by displacing warfarin from protein-binding sites. While several investigators have reported this interaction with use of high-dose methyl salicylate, this case indicates that a significant interaction can occur with the use of lower topical doses of methyl salicylate as well.

CONCLUSIONS

Healthcare providers and patients taking warfarin must be aware of the potential hazard of using topical methyl salicylate in combination with warfarin.

Sensitive and selective gas chromatographic methods for the quantitation of camphor, menthol and methyl salicylate from human plasma

Analytical methods using gas chromatography-flame ionization detection (GC-FID) for the quantitation of camphor and menthol and GC-MS for the quantitation of methyl salicylate have been developed for measurement of low concentrations from human plasma. Anethole serves as the internal standard for camphor and menthol and ethyl salicylate serves as the internal standard for methyl salicylate. Plasma samples undergo multiple, sequential extractions with hexane in order to provide optimal recovery. For menthol and camphor, the extracting solvent is reduced in volume and directly injected onto a capillary column (Simplicity-WAX). Extracted methyl salicylate is derivatized with BSTFA prior to injection onto a capillary column (Simplicity-5). Between-day variation (% RSD) at 5 ng/ml varies from 6.2% for methyl salicylate to 13.5% for camphor. The limit of detection for each analyte is 1 ng/ml and the limit of quantitation is 5 ng/ml. These analytical methods have been used in a clinical study to assess exposure from dermally applied patches containing the three compounds.

Self promotion of deep tissue penetration and distribution of methylsalicylate after topical application

PURPOSE

To determine how changes in cutaneous blood flow induced in-vivo by methylsalicylate (MeSA), compared to non-rubefacient triethanolamine salicylate (TSA), affected topical salicylate absorption and distribution, and to assess formulation therapeutic potential by comparing tissue concentrations to published antiinflammatory concentrations.

METHODS

Flux of salicylate from MeSA and TSA formulations applied to full-thickness rat skin was determined using in vitro diffusion cells. Anaesthetised rats were then used to quantify salicylate concentrations in plasma and tissues underlying the application site for the two formulations over a 6h period. In vitro and in vivo absorption profiles were then compared and the effect of MeSA on cutaneous blood flow assessed.

RESULTS

In vitro flux of salicylate from the MeSA formulation was 40% higher, though after correcting for differences in formulation concentrations the ratio of permeability coefficients was reversed. Contrary to the in vitro predictions, in vivo tissue and plasma concentrations of salicylate in rats rose rapidly in the first 1 hr and were more than the predicted 1.4-fold higher for MeSA. This effect was mirrored by the increase in blood flow induced by MeSA in human cutaneous vessels and that reported in the literature. Potential therapeutic levels were not seen below superficial muscle layers.

CONCLUSIONS

Direct tissue penetration of salicylate occurs below application sites from both MeSA and TSA formulations. Tissue concentrations of MeSA were higher than predicted due to its rapid distribution in the blood.

Serum concentrations of salicylic acid following topically applied salicylate derivatives

OBJECTIVE

To compare the rate and extent of systemic salicylate absorption following single and multiple applications of two topically applied analgesics, one containing methyl salicylate and the other containing trolamine salicylate.

DESIGN

Two-period, two-treatment, randomized, crossover, multiple-dose study in healthy men and women volunteers.

PARTICIPANTS

Six men and six women volunteers, 21-44 years of age.

INTERVENTIONS

Subjects applied 5 g of an ointment containing 12.5% methyl salicylate twice daily for 4 days (8 doses) or a cream containing trolamine 10% twice daily for two doses, to a 10-cm2 area on the thigh. Treatment order and leg (right or left) were assigned randomly. Subjects were crossed over to the alternate treatment on the other leg after a minimum washout period of 7 days.

MAIN OUTCOME MEASURES

The total amount of salicylate recovered in the urine during two dosing intervals (24 hours) on each study day, relative to the applied dose, was used to calculate the bioavailability of each product. Mean standard pharmacokinetic parameters including area under the curve, maximum concentration (Cmax), time to maximum concentration, and minimum concentrations at steady-state were determined from serum concentrations. Serum concentrations were fit to three pharmacokinetic models and the suitability of each model was evaluated. Estimates of absorption rate constant, clearance, volume, and fraction absorbed on day 1 were estimated by using the best-fitting model.

RESULTS

Salicylic acid could not be detected in serum after trolamine application. However, concentrations between 0.31 and 0.91 mg/L were detected within 1 hour of the first application of methyl salicylate and Cmax between 2 and 6 mg/L were observed following the seventh application on day 4. Both the extent and rate of absorption changed after the first 24 hours. The absorption rate constant increased significantly from the first to the seventh dose (first dose absorption rate constant: 0.16 h-1, seventh dose: 0.28 h-1; p < 0.035). Urinary recovery of total salicylate (salicylic acid and principal metabolites of salicylic acid) during the first 24 hours of the methyl salicylate phase averaged 175.2 mg, exceeding the 6.9 mg (p < 0.05) recovered during the trolamine phase. The recovery of salicylate in the urine in the first 24 hours after application of methyl salicylate was significantly greater than the 1.4% recovered after application of trolamine (p < 0.05). Furthermore, the fraction of methyl salicylate recovered in the urine increased significantly from 15.5% on day 1 to approximately 22% on the second, third, and fourth days.

CONCLUSIONS

A considerable amount of salicylic acid may be absorbed through the skin after topical application of methyl salicylate products and this may increase with multiple applications. Caution is warranted in patients for whom systemic salicylate may be hazardous or problematic.

Potential dangers from topical preparations containing methyl salicylate

Methyl salicylate (oil of wintergreen) is widely available in many over-the-counter liniments, ointments, lotions or medicated oils for the relief of musculoskeletal aches and pains. Ingestion of methyl salicylate poses the threat of severe, rapid-onset salicylate poisoning because of its liquid, concentrated form and lipid solubility. Excessive usage of these preparations in patients receiving warfarin may result in adverse interactions and bleedings. Methyl salicylate in topical analgesic preparations may cause irritant or allergic contact dermatitis and anaphylactic reactions. Physicians should fully appreciate the potential dangers from topical preparations containing methyl salicylate.

Adverse interactions between warfarin and nonsteroidal antiinflammatory drugs: mechanisms, clinical significance, and avoidance

OBJECTIVE

To review the mechanisms and clinical significance of adverse interactions between warfarin and nonsteroidal anti-inflammatory drugs (NSAIDs) and discuss how these interactions can be avoided.

DATA SOURCES

Previous studies of interactions between warfarin and NSAIDs or reports of adverse interactions were identified from a MEDLINE search (1976 to present) and from the reference lists of pertinent articles.

STUDY SELECTION AND DATA EXTRACTION

All articles were considered for inclusion in the review. Pertinent information was selected for discussion.

DATA SYNTHESIS

All NSAIDs can prolong bleeding time by inhibiting platelet function. High-dose aspirin has a direct hypoprothrombinemic effect. Phenylbutazone and its analogs enhance the hypoprothrombinemic effect of warfarin through a pharmacokinetic interaction by inhibiting the hepatic metabolism of warfarin. Mefenamic acid also enhances the anticoagulant effect of warfarin, but the mechanism is not known. The clinical relevance of protein binding displacement in the interaction between warfarin and NSAIDs has been overstated, although a significant one may be more likely in the presence of high concentrations of NSAIDs in patients with slow elimination of warfarin (e.g., those with severe heart failure or impaired liver function). NSAIDs can induce gastrointestinal bleeding, which is likely to be more severe if warfarin is also given.

CONCLUSIONS

The combined use of warfarin and NSAIDs is generally discouraged because of the increased risk of bleeding in these patients. In patients receiving warfarin who also require NSAIDs, phenylbutazone and its analogs, high-dose aspirin, mefenamic acid, excessive use of topical methyl salicylate, and NSAIDs that are associated with a higher risk of bleeding peptic ulcers should be avoided. Patients should be closely monitored for anticoagulant control and bleeding complications during the combined use of warfarin and NSAIDs.

Pharmacokinetic-pharmacodynamic drug interactions with nonsteroidal anti-inflammatory drugs

The nonsteroidal anti-inflammatory drugs (NSAIDs) are very commonly prescribed, especially in the elderly population. In many countries more than 10 different NSAIDs are available. As the older pyrazole compounds like phenylbutazone, oxyphenbutazone and azapropazone are most prone to pharmacokinetic interactions, the use of these compounds should be avoided where possible. Acidic NSAIDs interact with bile acid-binding resins, resulting in decreased concentrations of NSAIDs in the blood. In earlier reports it was suggested that the absorption of NSAIDs was affected by antacids and sucralfate. More recently, it was shown that there is delayed absorption of these drugs, but there is no difference in the extent of absorption. Only salicylates had their urinary secretion enhanced by antacids, which increase the urinary pH to values > 7. Histamine H2-receptor antagonists can be combined safely with NSAIDs. The concomitant administration of probenecid increased the blood concentration of NSAIDs, so an enhanced anti-inflammatory effect can be expected when these 2 drugs are combined. More importantly, NSAIDs can cause pharmacokinetic drug-drug interactions with other drugs. As can be expected, interactions with drugs that have a small therapeutic window are most likely to be of clinical significance. For example, lithium, medium to high dose methotrexate and, to a lesser extent, cyclosporin may be affected by concomitant administration of an NSAID. Aspirin (acetylsalicylic acid) and/or pyrazoles interact with oral anticoagulants, oral antihyperglycaemic agents and the anticonvulsants phenytoin and valproic acid (sodium valproate). Elevation of blood concentrations of these agents can be potentially dangerous. Similarly, NSAIDs interact with digoxin. This interaction is most likely to occur in the elderly, in neonates or in patients with renal impairment. Indomethacin can influence the blood concentrations of aminoglycosides in neonates. Unfortunately, this effect seems unpredictable, so practical therapeutic recommendations cannot be made. When NSAIDs are combined with salicylates or diflunisal, the blood concentrations of the salicylate or diflunisal may increase. However, the clinical relevance of this increase in drug concentration seems to be of minor importance. Gastrointestinal bleeding caused by NSAIDs is the most dangerous when it results from a mixed pharmacokinetic/pharmacodynamic interaction; however, patients are also at risk when pharmacodynamic interactions only are involved.

Clinically significant drug interactions with the oral anticoagulants

Oral anticoagulants were introduced in the late 1940s and remain widely used today. Indications include prevention of thrombosis associated with atrial fibrillation, structural cardiac diseases and following prosthetic valvular replacement. They have been used for both treatment and prophylaxis of deep venous thrombosis and in efforts to decrease the frequency and rate of second myocardial infarction. These compounds include the coumarin derivatives [dicoumarol (bishydroxycoumarin), phenprocoumon, nicoumalone (acenocoumarol)] and the indanedione derivatives (diphenadione, phenindione, anisindione) which, because of adverse reactions, are largely unavailable. The oral anticoagulants, and warfarin in particular, are highly interactive with other drugs. Mechanisms of those interactions include both pharmacokinetic and pharmacodynamic mechanisms and may result in either hyper- or hypoprothrombinaemia. Because their principal adverse reaction is haemorrhage, and interactions are widespread across many therapeutic specialties, it becomes imperative for the practising physician to be aware of the possibility of interaction whenever these agents are coadministered with other drugs.

Adverse interaction between warfarin and indomethacin

A 57-year-old man developed spontaneous skin bruising and haematuria during combined therapy with warfarin and indomethacin. Due to the potential effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on the anticoagulant action of warfarin and platelet function, patients receiving both warfarin and NSAIDs should have their prothrombin time monitored very closely. Also, the risk of gastrointestinal haemorrhage with any NSAIDs must always be remembered.