A review of the hepatotoxicity of paracetamol at therapeutic or near-therapeutic dose levels, with particular reference to alcohol abusers

The Unethical Use of Paracetamol As a Food Tenderizer in Four Selected African Countries: A Major Public Health Concern?

Paracetamol poisoning is the commonest cause of acute liver injury. Therefore, the unethical use of paracetamol as a food tenderizer poses a threat to human health. Although this is a common practice in Ghana, Uganda, Nigeria, and Kenya, there are few or no scientific records on the use of paracetamol as a food tenderizer and its deleterious effects, thus making it difficult to regulate this practice. This review aims to fully collate and present a systematic overview of the literature on the use of paracetamol as a food tenderizer in these countries, the potentially harmful effects posed by the practice, and measures in place to curb the situation. Additionally, this review aims to reveal the scientific gaps and areas requiring more research, thus providing a reference for further research to regulate this unscrupulous practice. From our extensive review of the literature, the high cost of fuel used in cooking and longer cooking times are the main reasons for the inappropriate use of paracetamol as a food tenderizer. Also, this review concludes that little has been done to create public awareness of this unethical practice. Furthermore, few ways to monitor, control and regulate this practice have been proposed.

Highly specific, sensitive and rapid enzyme immunoassays for the measurement of acetaminophen in serum

Acetaminophen antibodies were purified using affinity chromatography and labelled with horseradish peroxidase (HRP). The antibody-HRP conjugate and a new acetaminophen derivative were used in the construction of two immunoassay methods facilitating the direct quantitative measurement of acetaminophen in serum: a 96-well microtitre plate and coated-tube ELISAs. A minimum detection limit of 0.2 μg mL(-1) and a dynamic range of 0.2 to 1 μg mL(-1) in serum were achieved using the 96-well microtitre plate ELISA. The tube assay was optimised for the measurement of the clinically critical acetaminophen concentration of 50 to 250 μg mL(-1) of serum. The quantitative and specific tests could be completed within less than an hour. Common drugs including aspirin showed less than 0.1% cross-reactivity.

Age-related changes in the hepatic pharmacology and toxicology of paracetamol

Optimal pharmacotherapy is determined when the pharmacokinetics and pharmacodynamics of the drug are understood. However, the age-related changes in pharmacokinetics and pharmacodynamics, as well as the increased interindividual variation mean optimal dose selection are a challenge for prescribing in older adults. Poor understanding of how hepatic clearance and toxicity are different with age results in suboptimal dose selection, poor efficacy, and/or increased toxicity. Of particular concern is the analgesic paracetamol which has been in use for more than 50 years and is consumed by a large proportion of older adults. Paracetamol is considered to be a relatively safe drug; however, caution must be taken because of its potential for toxicity. Paracetamol-induced liver injury from accidental overdose accounts for up to 55% of cases in older adults. Better understanding of how age affects the hepatic clearance and toxicity of drugs will contribute to evidence-based prescribing for older people, leading to fewer adverse drug reactions without loss of benefit.

Alcoholic liver disease in the elderly

Although per capita alcohol consumption, and thus the prevalence of alcoholic liver disease, decreases generally with age in Europe and in the United States, recently an increase in alcohol consumption has been reported in individuals over 65 years. Reasons explaining this observation may include an increase in life expectancy or a loss of life partners and, thus, loneliness and depression. Although ethanol metabolism and ethanol distribution change with age, and an elderly person's liver is more susceptible to the toxic effect of ethanol, the spectrum of alcoholic liver diseases and their symptoms and signs is similar to that seen in patients of all ages. However, prognosis of alcoholic liver disease in the elderly is poor. In addition, chronic alcohol consumption may enhance drug associated liver disease and may also act as a cofactor in other liver diseases, such as viral hepatitis and nonalcoholic fatty liver disease.

Antituberculosis drugs and hepatotoxicity

Isoniazid, pyrazinamide and rifampicin have hepatotoxic potential, and can lead to such reactions during antituberculosis chemotherapy. Most of the hepatotoxic reactions are dose-related; some are, however, caused by drug hypersensitivity. The immunogenetics of antituberculosis drug-induced hepatotoxicity, especially inclusive of acetylaor phenotype polymorphism, have been increasingly unravelled. Other principal clinical risk factors for hepatotoxicity are old age, malnutrition, alcoholism, HIV infection, as well as chronic hepatitis B and C infections. Drug-induced hepatic dysfunction usually occurs within the initial few weeks of the intensive phase of antituberculosis chemotherapy. Vigilant clinical (including patient education on symptoms of hepatitis) and biochemical monitoring are mandatory to improve the outcomes of patients with drug-induced hepatotoxicity during antituberculosis chemotherapy. Some fluoroquinolones like ofloxacin/levofloxacin may have a role in constituting non-hepatotoxic drug regimens for management of tuberculosis (TB) in the presence of hepatic dysfunction. Isoniazid administration is currently the standard therapy for latent TB infection. Rifamycins like rifampicin or rifapentine, alone or in combination with isoniazid, may also be considered as alternatives, pending accumulation of further clinical data. During treatment of latent TB infection, regular follow up is essential to ensure adherence to therapy and facilitate clinical monitoring for hepatic dysfunction. Monitoring of liver chemistry is also required for those patients at risk of drug-induced hepatotoxicity.

Paracetamol, alcohol and the liver

It is claimed that chronic alcoholics are at increased risk of paracetamol (acetaminophen) hepatotoxicity not only following overdosage but also with its therapeutic use. Increased susceptibility is supposed to be due to induction of liver microsomal enzymes by ethanol with increased formation of the toxic metabolite of paracetamol. However, the clinical evidence in support of these claims is anecdotal and the same liver damage after overdosage occurs in patients who are not chronic alcoholics. Many alcoholic patients reported to have liver damage after taking paracetamol with 'therapeutic intent' had clearly taken substantial overdoses. No proper clinical studies have been carried out to investigate the alleged paracetamol-alcohol interaction and acute liver damage has never been produced by therapeutic doses of paracetamol given as a challenge to a chronic alcoholic. The paracetamol-alcohol interaction is complex; acute and chronic ethanol have opposite effects. In animals, chronic ethanol causes induction of hepatic microsomal enzymes and increases paracetamol hepatotoxicity as expected (ethanol primarily induces CYP2E1 and this isoform is important in the oxidative metabolism of paracetamol). However, in man, chronic alcohol ingestion causes only modest (about twofold) and short-lived induction of CYP2E1, and there is no corresponding increase (as claimed) in the toxic metabolic activation of paracetamol. The paracetamol-ethanol interaction is not specific for any one isoform of cytochrome P450, and it seems that isoenzymes other than CYP2E1 are primarily responsible for the oxidative metabolism of paracetamol in man. Acute ethanol inhibits the microsomal oxidation of paracetamol both in animals and man. This protects against liver damage in animals and there is evidence that it also does so in man. The protective effect disappears when ethanol is eliminated and the relative timing of ethanol and paracetamol intake is critical. In many of the reports where it is alleged that paracetamol hepatotoxicity was enhanced in chronic alcoholics, the reverse should have been the case because alcohol was actually taken at the same time as the paracetamol. Chronic alcoholics are likely to be most vulnerable to the toxic effects of paracetamol during the first few days of withdrawal but maximum therapeutic doses given at this time have no adverse effect on liver function tests. Although the possibility remains that chronic consumption of alcohol does increase the risk of paracetamol hepatotoxicity in man (perhaps by impairing glutathione synthesis), there is insufficient evidence to support the alleged major toxic interaction. It is astonishing that clinicians and others have unquestion-ingly accepted this supposed interaction in man for so long with such scant regard for scientific objectivity.

Pharmacokinetic interactions between alcohol and other drugs

The frequent use of alcohol (ethanol) together with prescription drugs gives any described pharmacokinetic interaction significant clinical implications. The issue is both the effect of alcohol on the pharmacokinetics of various drugs and also the effect of those drugs on the pharmacokinetics of alcohol. This review discusses these pharmacokinetic interactions but also briefly describes some other effects of alcohol that are clinically relevant to drug prescribing. The use of several different study designs may be required before we can confidently state the presence or absence of any alcohol-drug interaction. Short term administration of alcohol in volunteers is the most common study design but studies of social drinking and prolonged moderate alcohol intake can be important in some situations. Community-based studies may illustrate the clinical relevance of any interaction. Alcohol can affect the pharmacokinetics of drugs by altering gastric emptying or liver metabolism (by inducing cytochrome P450 2E1). Drugs may affect the pharmacokinetics of alcohol by altering gastric emptying and inhibiting gastric alcohol dehydrogenase. The role of gastric alcohol dehydrogenase in the first-pass metabolism of alcohol is reviewed in this article and the arguments for and against any potential interaction between alcohol and H2 receptor antagonists are also discussed. The inhibition of the metabolism of acetaldehyde may cause disulfiram-like reactions. Pharmacodynamic interactions between alcohol and prescription drugs are common, particularly the additive sedative effects with benzodiazepines and also with some of the antihistamine drugs; other interactions may occur with tricyclic antidepressants. Alcohol intake may be a contributing factor to the disease state which is being treated and may complicate treatment because of various pathophysiological effects (e.g. impairment of gluconeogenesis and the risk of hypoglycaemia with oral hypoglycaemic agents). The combination of nonsteroidal anti-inflammatory drugs and alcohol intake increases the risk of gastrointestinal haemorrhage.