Abstract

• Cannabis, Cannabis sativa L., is used to produce a resin that contains high levels of cannabinoids, particularly delta9-tetrahydrocannabinol (THC), which are psychoactive substances.
   
• Although cannabis use is illegal in France and in many other countries, it is widely used for its relaxing or euphoric effects, especially by adolescents and young adults. What are the adverse effects of cannabis on health? During consumption? And in the long term? Does cannabis predispose users to the development of psychotic disorders? To answer these questions, we reviewed the available evidence using the standard Prescrire methodology.
   
• The long-term adverse effects of cannabis are difficult to evaluate. Since it is an illegal drug, it is very difficult to obtain data about the use of cannabis and associated substances, with or without the user’s knowledge. Tobacco and alcohol consumption, and particular lifestyles and behaviours are often associated with cannabis use. Some traits predispose individuals to the use of psychoactive substances in general.
   
• The effects of cannabis are dose-dependent. The most frequently reported adverse effects are mental slowness, impaired reaction times, and sometimes accentuation of anxiety. Serious psychological disorders have been reported with high levels of intoxication.
   
• The relationship between poor school performance and early, regular, and frequent cannabis use seems to be a vicious circle, in which each sustains the other. Many studies have focused on the long-term effects of cannabis on memory, but their results have been inconclusive. There do not seem to be cognitive effects beyond one week after cannabis cessation.
   
• About fifteen longitudinal cohort studies that examined the influence of cannabis on depressive thoughts or suicidal ideation have yielded conflicting results and are inconclusive.
   
• Several longitudinal cohort studies have shown a statistical association between psychotic illness and self-reported cannabis use. However, the results are difficult to interpret due to methodological problems, particularly the unknown reliability of self-reported data. It has not been possible to establish a causal relationship in either direction, because of these methodological limitations.
   
• In Australia, the marked increase in cannabis use has not been accompanied by an increased incidence of schizophrenia.
   
• On the basis of the available data, we cannot reach firm conclusions on whether or not cannabis use causes psychosis. It seems prudent to inform apparently vulnerable individuals that cannabis may cause acute psychotic decompensation, especially at high doses.
   
• Users can feel dependent on cannabis, but this dependence is usually psychological. Withdrawal symptoms tend to occur within 48 hours following cessation of regular cannabis use, and include increased irritability, anxiety, nervousness, restlessness, sleep difficulties and aggression. Symptoms subside within 2 to 12 weeks.
   
• Driving under the influence of cannabis doubles the risk of causing a fatal road accident. Alcohol consumption plays an even greater role. Simultaneous use of cannabis and alcohol markedly increases the risk.
   
• A few studies and a number of isolated reports suggest that cannabis has a role in the occurrence of cardiovascular adverse effects, especially in patients with coronary heart disease.
   
• Numerous case-control studies have investigated the role of cannabis in the incidence of some types of cancer. Its role has not been ruled out, but it is not possible to determine whether the risk is distinct from that of the tobacco with which it is often smoked.
   
• Studies that have examined the influence of cannabis use on the clinical course of hepatitis C are inconclusive. Alcohol remains the main toxic agent that hepatitis C patients should avoid.
   
• In practice, the adverse effects of low-level, recreational cannabis use are generally minor, although they can apparently be serious in vulnerable individuals. The adverse effects of cannabis appear overall to be less serious than those of alcohol, in terms of neuropsychological and somatic effects, accidents and violence. 

©Prescrire January 2011

"Adverse effects of cannabis" Prescrire Int 2011; 20 (112): 18-23. (Pdf, subscribers only)