Review article
Adverse effects of cannabis on health: an update of the literature since 1996

In tribute to the memory of the late Professor Corneille Radouco-Thomas, friend, scientific colleague, and distinguished educator.
https://doi.org/10.1016/j.pnpbp.2004.05.027Get rights and content

Abstract

Recent research has clarified a number of important questions concerning adverse effects of cannabis on health. A causal role of acute cannabis intoxication in motor vehicle and other accidents has now been shown by the presence of measurable levels of Δ9-tetrahydrocannabinol (THC) in the blood of injured drivers in the absence of alcohol or other drugs, by surveys of driving under the influence of cannabis, and by significantly higher accident culpability risk of drivers using cannabis. Chronic inflammatory and precancerous changes in the airways have been demonstrated in cannabis smokers, and the most recent case-control study shows an increased risk of airways cancer that is proportional to the amount of cannabis use. Several different studies indicate that the epidemiological link between cannabis use and schizophrenia probably represents a causal role of cannabis in precipitating the onset or relapse of schizophrenia. A weaker but significant link between cannabis and depression has been found in various cohort studies, but the nature of the link is not yet clear. A large body of evidence now demonstrates that cannabis dependence, both behavioral and physical, does occur in about 7–10% of regular users, and that early onset of use, and especially of weekly or daily use, is a strong predictor of future dependence. Cognitive impairments of various types are readily demonstrable during acute cannabis intoxication, but there is no suitable evidence yet available to permit a decision as to whether long-lasting or permanent functional losses can result from chronic heavy use in adults. However, a small but growing body of evidence indicates subtle but apparently permanent effects on memory, information processing, and executive functions, in the offspring of women who used cannabis during pregnancy. In total, the evidence indicates that regular heavy use of cannabis carries significant risks for the individual user and for the health care system.

Introduction

Since the publication of the Indian Hemp Drugs Commission Report of 1893–1894 Indian Hemp Drugs Commission, 1894, Kalant, 1972, there have been many excellent reviews of the literature on health effects of cannabis, both harmful and potentially beneficial. One of the most extensive recent reviews, carried out by a WHO ad hoc Working Group in 1994–1996 (Kalant et al., 1999), was based on a series of commissioned reviews of the literature published up to 1996. The present update is therefore based on a review of the literature from 1996 to the present day, as identified in Medline and in the monthly research notes of the Canadian Consortium for the Clinical Investigation of Cannabis.

During the past 8 years, there has been a considerable increase in the volume of research literature on cannabis and cannabinoids, but much of this increase has been in the area of potential therapeutic uses of cannabinoids, both natural and synthetic, and in the area of the physiological roles of the endocannabinoid system (the compounds formed naturally in the mammalian body that act upon the cannabinoid receptors in brain and other organs). Much of the research in experimental animals also deals with molecular mechanisms of action and with neurobiology of cannabis. Much of this material has been discussed in thorough reviews elsewhere, and is not covered here, since it is not directly relevant to the question of adverse effects arising from the non-medical use of cannabis.

The great bulk of the recent literature dealing with these adverse effects in humans has focused on three topics: cannabis and driving accidents, dependence on cannabis, and psychiatric effects of cannabis use. Significant but smaller amounts of the recent literature deal with pulmonary effects, cancer, toxicity, and cognitive function. These various topics are reviewed separately below.

Section snippets

Driving ability

A thorough review of the literature on the effects of cannabis on driving skills and driving safety was published by Smiley (1999), originally prepared as a background paper for the WHO report but later updated to 1999 for publication in the separate volume of background papers (Kalant et al., 1999). She concluded that experimental studies have shown clear but modest impairment of driving skills and actual driving performance in subjects smoking small or moderate doses of cannabis, but that the

Summary

This review of the literature since 1996, concerning adverse health effects of cannabis smoking, does not alter in a major way the conclusions reached in an earlier review of the literature up to 1996. The principal additions to, or modifications of, those earlier conclusions are as follows:

  • The potential role of cannabis in driving accidents has been further supported by analytical data showing the presence of THC in the blood and saliva of injured or impaired drivers with a higher frequency

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