Cannabis-related allergies: An international overview and consensus recommendations

IgE-Mediated Cannabis Allergy and Cross-Reactivity Syndromes: A Roadmap for Correct Diagnosis and Management

PURPOSE OF THE REVIEW

With increased access and decriminalization of cannabis use, cases of IgE-dependent cannabis allergy (CA) and cross-reactivity syndromes have been increasingly reported. However, the exact prevalence of cannabis allergy and associated cross-reactive food syndromes (CAFS) remains unknown and is likely to be underestimated due to a lack of awareness and insufficient knowledge of the subject among health care professionals. Therefore, this practical roadmap aims to familiarize the reader with the early recognition and correct management of IgE-dependent cannabis-related allergies. In order to understand the mechanisms underlying these cross-reactivity syndromes and to enable personalized diagnosis and management, special attention is given to the molecular diagnosis of cannabis-related allergies.

RECENT FINDINGS

The predominant signs and symptoms of CA are rhinoconjunctivitis and contact urticaria/angioedema. However, CA can also present as a life-threatening condition. In addition, many patients with CA also have distinct cross-reactivity syndromes, mainly involving fruits, vegetables, nuts and cereals. At present, five allergenic components of Cannabis sativa (Can s); Can s 2 (profilin), Can s 3 (a non-specific lipid protein), Can s 4 (oxygen-evolving enhancer protein 2 oxygen), Can s 5 (the Bet v 1 homologue) and Can s 7 (thaumatin-like protein) have been characterized and indexed in the WHO International Union of Immunological Sciences (IUIS) allergen database. However, neither of them is currently readily available for diagnosis, which generally starts by testing crude extracts of native allergens. The road to a clear understanding of CA and the associated cross-reactive food syndromes (CAFS) is still long and winding, but well worth further exploration.

Regulatory Framework for Cannabis: A Position Paper From the American College of Physicians

Cannabis, also known as marijuana, is the dried flowers, stems, seeds, and leaves of the Cannabis sativa plant. It contains more than 100 compounds, including tetrahydrocannabinol, which has psychoactive effects. Federal law prohibits the possession, distribution, and use of cannabis outside limited research activities, but most states have legalized cannabis for medical or recreational use. However, research into the potential therapeutic and adverse health effects of cannabis has been limited, in part because of the drug's federal legal status. In this position paper, the American College of Physicians (ACP) calls for the decriminalization of possession of small amounts of cannabis for personal use and outlines a public health approach to controlling cannabis in jurisdictions where it is legal. ACP recommends the rigorous evaluation of the health effects and potential therapeutic uses of cannabis and cannabinoids as well as research into the effects of legalization on cannabis use. It also calls for evidence-based medical education related to cannabis and increased resources for treatment of cannabis use disorder.

Bioaerosol Exposures and Respiratory Diseases in Cannabis Workers

PURPOSE OF REVIEW

This review investigates occupational inhalation hazards associated with biologically derived airborne particles (bioaerosols) generated in indoor cannabis cultivation and manufacturing facilities.

RECENT FINDINGS

Indoor cannabis production is growing across the US as are recent reports of respiratory diseases among cannabis workers, including occupational asthma morbidity and mortality. More information is needed to understand how bioaerosol exposure in cannabis facilities impacts worker health and occupational disease risk. Preliminary studies demonstrate a significant fraction of airborne particles in cannabis facilities are comprised of fungal spores, bacteria, and plant material, which may also contain hazardous microbial metabolites and allergens. These bioaerosols may pose pathogenic, allergenic, toxigenic, and pro-inflammatory risks to workers. The absence of multi-level, holistic bioaerosol research in cannabis work environments necessitates further characterization of the potential respiratory hazards and effective risk prevention methods to safeguard occupational health as the cannabis industry continues to expand across the US and beyond.

IgE-Mediated Allergy and Asymptomatic Sensitization to Cannabis Allergens-Review of Current Knowledge and Presentation of Six Cases

Cannabis allergy is a relatively new phenomenon described in the 1970s. Its increased frequency has been observed over the last years due to the increasing therapeutic and recreational use of cannabis-based products. Sensitization possibly leading to allergy symptoms can occur not only through the smoking of cannabis, but also through ingestion, the inhalation of pollen, or direct contact. The severity of symptoms varies from benign pruritus to anaphylaxis. There is scant information available to support clinicians throughout the entire therapeutic process, starting from diagnosis and ending in treatment. In this review, we present six cases of patients in whom molecular in vitro testing revealed sensitization to cannabis extract and/or cannabis-derived nsLTP molecules (Can s 3). Based on these cases, we raise important questions regarding this topic. The article discusses current proposals and highlights the importance of further research not only on cannabis allergy but also on asymptomatic sensitization to cannabis allergens, which may be ascertained in some percentage of the population.

Emerging and Novel Elicitors of Anaphylaxis: Collegium Internationale Allergologicum Update 2024

BACKGROUND

Anaphylaxis represents the most severe end of the spectrum of allergic reactions. Frequent elicitors of anaphylaxis are insects, foods, and drugs. This paper summarizes recent development with regard to emerging and novel elicitors of anaphylaxis.

SUMMARY

Food allergens on the rise include pulses (like pea, chickpea), seeds (hemp, chia), nuts (cashew), pseudograins (buckwheat, quinoa), fruits, and microalgae. Novel foods are foods that were not consumed to any significant extent in the European Union before May 1997, which includes four edible insects (mealworm, migratory locust, house cricket, and buffalo worm). Recent investigations have pointed out the risk of anaphylaxis associated with the consumption of yellow mealworm for people allergic to shellfish and house dust mites. In Europe, fire ants (mostly Solenopsis invicta) and Vespa velutina nigrithorax represent invasive species, which account for increasing numbers of anaphylactic reactions. Also, several new drugs, especially biologicals, have been associated with anaphylaxis.

KEY MESSAGES

Elicitors of anaphylaxis are changing as a result of (i) increase in demand for plant-based food, (ii) introduction of novel foods, (iii) spreading of allergens by climate changes and globalization, or (iv) due to exposure to newly developed drugs.

Imported allergens in Italy: an emerging issue

Imported allergens are involved in many allergic reactions, with unexpected and unusual implications. They can be involved in developing asthma, allergic rhinoconjunctivitis, Hymenoptera venom allergies and food allergies. Imported allergens can be implied in respiratory allergies attributable to commercial practices and accidental diffusion through air currents that have introduced non-native species in new geographical contexts. Ambrosia artemisiifolia L., a plant native to North America and currently in the western part of Lombardy, represents an example. Moreover, a variation in the pollen concentration in the Northwest Tuscany area and Trentino Alto-Adige was observed. Cannabis sativa is another imported allergen used frequently by adolescents. Regarding potential imported food allergens, there is no validated list. Imported food allergens derive from ethnic foods, referring to Mexican/Latin American, Chinese/Japanese, Southeast Asian, Arab/Middle Eastern and African cuisine. Four insect flours were recently introduced to the European and Italian markets (Acheta domesticus, Alphitobius diaperinus, Tenebrio molitor and Locusta migratoria). The association between the accidental introduction through commercial traffic, climate change, and the absence of natural enemies in the destination ecosystem is related to the introduction of a specific Hymenoptera, Vespa velutina, in Italy and Europe. External events attributable to human activities, such as climate change and the introduction of non-native plants, foods and Hymenoptera through trade, have contributed to the issue of imported allergens. Making the correct diagnosis and guiding the diagnostic and therapeutic path in this particular context represent the concerns of the pediatric allergist.

Allergy to Gibberellin-regulated proteins in an adolescent: A case of orange-induced anaphylaxis mediated by cofactors

This report is a case of anaphylaxis in an adolescent due to allergy to gibberellin-regulated proteins mediated by cofactors, in probable relation to a pollen/food allergy syndrome. It should also emphasizes the importance of obtaining a faithful clinical history, especially when it comes to adolescent patients as they tend to initiate toxic habits.

Feast for thought: A comprehensive review of food allergy 2021-2023

A review of the latest publications in food allergy over the past couple of years confirmed that food allergy is a major public health concern, affecting about 8% of children and 10% of adults in developed countries. The prevalence of food allergy varies around the world, with the increase being driven mainly by environmental factors, possibly together with genetic susceptibility to environmental changes. A precise diagnosis of food allergy is extremely important. Both new tests (eg, the basophil activation test) and improved optimization of information provided by existing tests (eg, the skin prick test and measurement of specific IgE level) can contribute to improving the accuracy and patients' comfort of food allergy diagnosis. Understanding the underlying immune mechanisms is fundamental to designing allergen-specific treatments that can be safe and effective in the long term. New discoveries of the immune response to food allergens, including T-cell and B-cell responses, have emerged. Novel therapeutic approaches are being trialed at various stages of development as attempts to allow for more active intervention to treat food allergy. Prevention is key to reducing the increase in prevalence. Early introduction of allergenic foods seems to be the most effective intervention, but others are being studied, and will, it is hoped, lead to modification of the epidemiologic trajectory of food allergy over time.

Concerns Related to the Consequences of Pediatric Cannabis Use: A 360-Degree View

Cannabis, a plant known for its recreational use, has gained global attention due to its widespread use and addiction potential. Derived from the Cannabis sativa plant, it contains a rich array of phytochemicals concentrated in resin-rich trichomes. The main cannabinoids, delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD), interact with CB1 and CB2 receptors, influencing various physiological processes. Particularly concerning is its prevalence among adolescents, often driven by the need for social connection and anxiety alleviation. This paper provides a comprehensive overview of cannabis use, its effects, and potential health risks, especially in adolescent consumption. It covers short-term and long-term effects on different body systems and mental health and highlights the need for informed decision making and public health initiatives, particularly regarding adolescent cannabis use.

Epicutaneous Sensitization to the Phytocannabinoid β-Caryophyllene Induces Pruritic Inflammation

In recent years, there has been increased accessibility to cannabis for recreational and medicinal use. Incidentally, there has been an increase in reports describing allergic reactions to cannabis including exacerbation of underlying asthma. Recently, multiple protein allergens were discovered in cannabis, yet these fail to explain allergic sensitization in many patients, particularly urticaria and angioedema. Cannabis has a rich chemical profile including cannabinoids and terpenes that possess immunomodulatory potential. We examined whether major cannabinoids of cannabis such as cannabidiol (CBD) and the bicyclic sesquiterpene beta-caryophyllene (β-CP) act as contact sensitizers. The repeated topical application of mice skin with β-CP at 10 mg/mL (50 µL) induced an itch response and dermatitis at 2 weeks in mice, which were sustained for the period of study. Histopathological analysis of skin tissues revealed significant edema and desquamation for β-CP at 10 mg/mL. For CBD and β-CP, we observed a dose-dependent increase in epidermal thickening with profound thickening observed for β-CP at 10 mg/mL. Significant trafficking of CD11b cells was observed in various compartments of the skin in response to treatment with β-CP in a concentration-dependent manner. Mast cell trafficking was restricted to β-CP (10 mg/mL). Mouse proteome profiler cytokine/chemokine array revealed upregulation of complement C5/5a (anaphylatoxin), soluble intracellular adhesion molecule-1 (sICAM-1) and IL-1 receptor antagonist (IL-1RA) in animals dosed with β-CP (10 mg/mL). Moreover, we observed a dose-dependent increase in serum IgE in animals dosed with β-CP. Treatment with β-CP (10 mg/mL) significantly reduced filaggrin expression, an indicator of barrier disruption. In contrast, treatment with CBD at all concentrations failed to evoke scratching and dermatitis in mice and did not result in increased serum IgE. Further, skin tissues were devoid of any remarkable features, although at 10 mg/mL CBD we did observe the accumulation of dermal CD11b cells in skin tissue sections. We also observed increased filaggrin staining in mice repeatedly dosed with CBD (10 mg/mL). Collectively, our studies indicate that repeated exposure to high concentrations of β-CP can induce dermatitis-like pathological outcomes in mice.

Proteomics-Based Approach for Detailing the Allergenic Profile of Cannabis Chemotypes

Allergic sensitization to cannabis is an emerging public health concern and is difficult to clinically establish owing to lack of standardized diagnostic approaches. Attempts to develop diagnostic tools were largely hampered by the Schedule I restrictions on cannabis, which limited accessibility for research. Recently, however, hemp was removed from the classified list, and increased accessibility to hemp allows for the evaluation of its practical clinical value for allergy diagnosis. We hypothesized that the proteomic profile is preserved across different cannabis chemotypes and that hemp would be an ideal source of plant material for clinical testing. Using a proteomics-based approach, we examined whether distinct varieties of cannabis plant contain relevant allergens of cannabis. Cannabis extracts were generated from high tetrahydrocannabinol variety (Mx), high cannabidiol variety (V1-19) and mixed profile variety (B5) using a Plant Total Protein Extraction Kit. Hemp extracts were generated using other standardized methods. Protein samples were subjected to nanoscale tandem mass spectrometry. Acquired peptides sequences were examined against the Cannabis sativa database to establish protein identity. Non-specific lipid transfer protein (Can s 3) level was measured using a recently developed ELISA 2.0 assay. Proteomic analysis identified 49 distinct potential allergens in protein extracts from all chemotypes. Most importantly, clinically relevant and validated allergens, such as profilin (Can s 2), Can s 3 and Bet v 1-domain-containing protein 10 (Can s 5), were identified in all chemotypes at label-free quantification (LFP) intensities > 106. However, the oxygen evolving enhancer protein 2 (Can s 4) was not detected in any of the protein samples. Similarly, Can s 2, Can s 3 and Can s 5 peptides were also detected in hemp protein extracts. The validation of these findings using the ELISA 2.0 assay indicated that hemp extract contains 30-37 ng of Can s 3 allergen per µg of total protein. Our proteomic studies indicate that relevant cannabis allergens are consistently expressed across distinct cannabis chemotypes. Further, hemp may serve as an ideal practical substitute for clinical testing, since it expresses most allergens relevant to cannabis sensitization, including the validated major allergen Can s 3.

Hemp seed: An allergen source with potential cross-reactivity to hazelnut

The increasing exposure of the population to Cannabis sativa has revealed allergies to different parts of the plant, among which hemp seed. Nonetheless, the major hemp seed allergens remain to be identified. Several known families of allergens are present in hemp seed, including notably seed storage proteins. We therefore aimed to investigate the potential allergenicity of the hemp seed storage proteins and their potential cross-reactivity to different seeds and nuts. For this, we extracted hemp seed proteins sequentially using buffers with increasing levels of salinity (H₂O, T2 and T3) to yield extracts differentially enriched in storage proteins. We used these extracts to perform immunoblots and ELISAs using sera of patients either sensitized to hemp seeds or sensitized/allergic to other seeds and nuts. Immunoblots and proteomics analyses identified vicilins and edestins as potential hemp seed allergens. Moreover, ELISA analyses revealed a correlation between sensitization to hazelnut and the hemp seed T3 extract (enriched in storage proteins). The possible cross-reactivity between hazelnut and hemp seed proteins was further strengthened by the results from inhibition ELISAs: the incubation of sera from hazelnut-sensitized individuals with increasing concentrations of the T3 extract inhibited serum IgE binding to the hazelnut extract by about 25-30%. Our study thus identifies vicilins and edestins as potential hemp seed allergens and highlights a possible cross-reactivity with hazelnut. The clinical relevance of this cross-reactivity between hemp seed and hazelnut needs to be further investigated in hazelnut-allergic individuals.

The Emerging Spectrum of Respiratory Diseases in the U.S. Cannabis Industry

While the cannabis industry is one of the fastest growing job markets in the United States and globally, relatively little is known about the occupational hazards that cannabis production workers face. Based on the closely related hemp industry and preliminary studies from recreational cannabis grow facilities, there is concern for significant respiratory exposures to bioaerosols containing microbial and plant allergens, chemicals such as pesticides, volatile organic compounds, and other irritant gases. Components of the cannabis plant have also recently been identified as allergenic and capable of inducing an immunoglobulin E-mediated response. Accumulating evidence indicates a spectrum of work-related respiratory diseases, particularly asthma and other allergic diseases. Disentangling causal relationships is difficult given the heterogeneity of mixed exposures, diagnostic challenges, and confounding by personal cannabis use. Despite and because of these uncertainties, better regulatory guidance and exposure controls need to be defined in order to reduce the risk of work-related disease.

Vegan diets from an allergy point of view - Position paper of the DGAKI working group on food allergy

Vegan diets are currently attracting a great deal of attention. However, avoiding animal-based foods restricts the diet and is associated with risks, the extent and medical implications of which are at present not sufficiently understood. Elimination diets represent the usual therapeutic long-term management in the presence of food allergy. In order to understand the risks of vegan diets and to discuss these critically from the perspective of food allergies, the expertise of a nutritionist/dietitian with expertise in this area is indispensable. This position paper deals with the incentives behind and the benefits of a plant-based diet. The knowledge required to cover macro- and micronutrient dietary requirements is presented. Using the avoidance of cow's milk as an example, the challenges of adequately meeting nutritional needs are identified and (so-called) milk alternatives are evaluated from an allergy and nutritional point of view. Finally, other plant-based (substitute) products are evaluated from the same perspective, as significant protein sources in vegan diets (e.g., legumes, nuts, and seeds) are at the same time potential and potent triggers of allergic reactions. However, the allergic potential of many substitute products cannot be fully assessed at present due to gaps in research. Wheat as the most important trigger for anaphylaxis in adults is also evaluated. Finally, the increase in ultra-processed products in the (vegan) food sector and their potential consequences for the immune system are discussed.

A review of cannabis allergy in the early days of legalization

Cannabis allergy is a burgeoning field; consequently, research is still in its infancy and allergists' knowledge surrounding this topic is limited. As cannabis legalization expands across the world, it is anticipated that there will be an increase in cannabis use. Thus, we hypothesize that a concomitant rise in the incidence of allergy to this plant can be expected. Initiatives aimed at properly educating health care professionals are therefore necessary. This review presents the most up-to-date information on a broad range of topics related to cannabis allergy. Although the clinical features of cannabis allergy are becoming more well described and recognized, the tools available to make a correct diagnosis are meager and often poorly accessible. In addition, research on cannabis allergy is still taking its first steps, and new and potentially groundbreaking findings in this field are expected to occur in the next few years. Finally, although therapeutic approaches are being developed, patient and physician education regarding cannabis allergy is certainly needed.

The epidemiology of food allergy in adults

The prevalence and awareness of food allergy (FA) among US adults is arguably at a historical high, both with respect to primary immunoglobulin E-mediated food hypersensitivity and other food-triggered conditions that operate through a variety of immunologic mechanisms (eg, pollen-FA syndrome, alpha-gal syndrome, food protein-induced enterocolitis syndrome, eosinophilic esophagitis). Worryingly, not only are many adults retaining childhood-onset food allergies as they age into adulthood, it seems that many adults are experiencing adult-onset allergies to previously tolerated foods, with correspondingly adverse physical, and psychological health impacts. Consequently, this review aims to summarize what is currently known about the epidemiology and population-level burden of FA among adult populations in North America and around the globe. This article also provides insights into the natural history of these conditions and what we need to know as we look to the future to support effective care and prevent FA.

Food anaphylaxis in the elderly: Analysis of allergy vigilance network data from 2002 to 2021

BACKGROUND

Few studies have focused on food allergies in the elderly, even though it may persist or appear de novo.

METHODS

We reviewed data for all cases of food-induced anaphylaxis in people age ≥ 60 reported to the French "Allergy Vigilance Network" (RAV) between 2002 and 2021. RAV collates data reported by French-speaking allergists regarding cases of anaphylaxis graded II to IV according to the Ring and Messmer classification.

RESULTS

In total, 191 cases were reported, with an even sex distribution and mean age was 67.4 years (range 60 to 93). The most frequent allergens were mammalian meat and offal (31 cases, 16.2%), often associated with IgE to α-Gal. Legumes were reported in 26 cases (13.6%), fruits and vegetables in 25 cases (13.1%), shellfish 25 cases (13.1%), nuts 20 cases (10.5%), cereals 18 cases (9.4%), seeds 10 cases (5.2%), fish 8 cases (4.2%) and anisakis 8 cases (4.2%). Severity was grade II in 86 cases (45%), grade III in 98 cases (52%) and grade IV in 6 cases (3%) with one death. Most episodes occurred at home or in a restaurant and in most cases adrenaline was not used to treat the acute episode. Potentially relevant cofactors such as beta-blocker, alcohol or non-steroidal anti-inflammatory drug intake were present in 61% of cases. Chronic cardiomyopathy, present in 11.5% of the population, was associated with greater, grade III or IV reaction severity (OR 3.4; 1.24-10.95).

CONCLUSION

Anaphylaxis in the elderly has different causes to younger people and requires detailed diagnostic testing and individualized care plans.

Immunomodulatory Actions of Cannabinoids: Clinical Correlates and Therapeutic Opportunities for Allergic Inflammation

Endogenously produced cannabinoids as well as phytocannabinoids broadly exhibit anti-inflammatory actions. Recent emergence of cannabis for multiple medical issues combined with reports on potent immunomodulatory actions of distinct components has underscored the therapeutic potential of cannabis. Although synthetic cannabinoids that are based on structural similarities to the existing class of cannabinoids have been on the rise, their application in therapeutics have been limited owing to toxicity concerns. Herein, we review the current literature that details the immunomodulatory actions of cannabinoids. Further, we highlight the complexities of cannabinoid biology and examine the potential inflammatory risks associated with the use of cannabis including potential for toxic interactions between distinct constituents of cannabis.

Anaphylaxis: Advances in the Past 10 Years

In the past 10 years, anaphylaxis has grown into its own special area of study within Allergy-Immunology, both at the bench and at the bedside. This review focuses on some of the most clinically relevant advances over the past decade. These include simplified and more inclusive diagnostic criteria for adults and children, uniform definition of biphasic anaphylaxis, and improved systems for objective severity grading. Studies reported in the past decade have led to improved understanding of normal and abnormal regulation of mast cell function, translating into better diagnostic and therapeutic approaches to patients with anaphylaxis. Research has provided improved recognition and treatment of mast cell disorders and has identified a new condition, hereditary α-tryptasemia, that may impact anaphylactic syndromes. We have learned to recognize new causes (α-gal), new pathways (Mas-related G protein-coupled receptor-X2), and many risk factors for severe anaphylaxis. The stability of epinephrine in autoinjectors was reported to be very good for several years after the labeled expiry date, and it can tolerate freezing and thawing. Repeated and prolonged exposure to excessive heat leads to degradation of epinephrine activity. New treatments to prevent severe anaphylaxis have been described, using new ways to block the IgE receptor or modulate intracellular signaling pathways.

Establishing diagnostic strategies for cannabis allergy

INTRODUCTION

Cannabis is the most widely consumed illicit drug in the world and carries a risk of severe IgE-mediated allergic reactions, requiring appropriate diagnostic management. Currently available diagnostics are still relatively limited and require careful interpretation of results to avoid harmful over- and underdiagnosis.

AREAS COVERED

This review focuses on the most up-to-date understandings of cannabis allergy diagnosis, starting with the main clinical features of the disease and the allergenic characteristics of Cannabis sativa, and then providing insights into in vivo, in vitro, and ex vivo diagnostic tests.

EXPERT OPINION

At present, the diagnosis of IgE-mediated cannabis allergy is based on a three-step approach that starts with accurate history taking and ends with a confirmation of sensitization to the whole extract and, finally, molecular components. Although much has been discovered since its first description in 1971, the diagnosis of cannabis allergy still has many unmet needs. The lack of commercial standardized and validated extracts and in vitro assays makes a harmonized workup of cannabis allergy difficult. Furthermore, the epidemiological characteristics, and clinical implications of sensitization to different molecular components are not yet fully known. Future research will complete the picture and likely result in an individualized and standardized approach.

Inhaled Marijuana and the Lung

Although vaping has recently increased as a mode of inhaling marijuana and has been associated with numerous and sometimes fatal cases of acute severe lung injury, smoking remains the most common method of inhaling marijuana and has been studied more extensively. Smoking marijuana has been shown to produce modest but significant short-term bronchodilation both in healthy subjects and those with asthma. Long-term effects of habitual marijuana smoking include the following: 1) symptoms of chronic bronchitis (increased cough, sputum production and wheezing); 2) modest effects on lung function in cross-sectional studies (no significant decrease in forced expired volume in 1 second [FEV₁) but mild reductions in FEV₁/forced vital capacity ratio [FEV₁/FVC], an increase in FVC and other lung volumes, reductions in specific airway conductance, and variable effects of maximal mid-expiratory flow rates and diffusing capacity; and 3) variable effects on age-related decline in FEV1 in longitudinal studies. Most cohort and case-control studies have failed to show that marijuana smoking is a significant risk-factor for lung cancer despite the presence of pro-carcinogenic components in marijuana smoke, although further study is warranted. The question whether marijuana smoking is associated with asthma is unclear and requires further investigation. Although delta-9 tetrahydrocannabinol (THC), the principal psychoactive component of marijuana, has immunomodulatory properties that hypothetically could increase the risk of pneumonia, the few available studies in marijuana smokers have failed to find an increased risk of pneumonia in immunocompetent users, although effects in immunosuppressed individuals have been variable.