We searched PubMed and Thompson Reuters Web of Science citation indexes for articles published in the past 10 years on adverse health effects of cannabis, with the search terms “cannabis”, “marijuana abuse”, “marijuana smoking”, “epidemiologic studies”, “adverse effects”, “substance related disorders”, “lung diseases”, “respiration disorders”, “cardiovascular diseases”, “coronary disease”, “traffic accidents”, “automobile driving”, “mental disorders”, and “adolescent”. Most selected
ReviewAdverse health effects of non-medical cannabis use
Introduction
Psychoactive preparations of Cannabis sativa have been used for over 4000 years for medical and religious purposes.1 Over the past 50 years, they have been increasingly adopted by adolescents and young adults for recreational use—in social settings to increase sociability and produce euphoric and intoxicating effects. Since cannabis use was first reported over 40 years ago by US college students, its recreational or non-medical use has spread globally, first to high-income countries, and recently to low-income and middle-income countries2, 3 (Figure 1, Figure 2).
Uncertainties exist about the number of people who use cannabis because of lack of timely, good-quality data in most countries. The UN Office on Drugs and Crime has estimated that in 2006 cannabis was used by 166 million adults (3·9% of the global population aged 15–64 years).4 Use was the highest in the USA, Australia, and New Zealand, followed by Europe. These countries reported higher rates of cannabis use than did the Middle East and Asia.4 Some African countries are also thought to have high rates of cannabis use.4 Because of their large populations, 31%, 25%, and 24% of the world's cannabis users are estimated to be from Asia, Africa, and the Americas, respectively, compared with 18% in Europe and 2% in Oceania4 (figure 1).
Section snippets
Pattern of cannabis use
In the USA, rates of cannabis use in young adults peaked in 1979, which was followed by a long decline until the early 1990s, when use increased again, before levelling off towards the end of the decade.5 A similar rise in its use in the early 1990s, followed by decline or stabilisation in recent years, has been reported in Australia and western Europe.5
Research in the USA has indicated that about 10% of those who ever use cannabis become daily users, and 20% to 30% become weekly users.5
Cannabis
The effects of cannabis depend on the dose received, the mode of administration, the user's previous experience with this drug, and the set and setting—ie, the user's expectations, attitudes towards the effects of cannabis, the mood state, and the social setting in which it is used.5 The main reason why most young people use cannabis is to experience a so-called high: mild euphoria, relaxation, and perceptual alterations, including time distortion and intensification of ordinary experiences
Health effects of cannabis
We looked for evidence: that an association exists between cannabis use and outcomes in case–control and prospective studies; that reverse causation was an implausible explanation of the association (evidence from prospective studies that cannabis use preceded the outcome); from prospective studies that controlled for potential confounding variables (such as other drug use and characteristics on which cannabis users differed from non-users); and that a causal association was biologically
Other illicit drug use
In the USA, Australia, and New Zealand, regular cannabis users were most likely to later use heroin and cocaine, and the earlier the age at which a young person uses cannabis, the more likely they are to use heroin and cocaine.71 Three explanations have been given for these patterns of drug involvement: cannabis users have more opportunities to use other illicit drugs because cannabis is supplied by the same black market; those who are early cannabis users are more likely to use other illicit
Cannabis and mental health
Cannabis use has been associated with increased risk of psychiatric disorders. A 15-year follow-up of 50 465 Swedish male conscripts reported that those who had tried cannabis by age 18 years were 2·4 times more likely to be diagnosed with schizophrenia than those who had not.81 Risk increased with the frequency of cannabis use and remained significant after statistical adjustment for a few confounding variables. Those who had used cannabis ten or more times by 18 years of age were 2·3 times
Increased THC content in cannabis products
Concerns have been expressed over the past 20 years about putative increases in the potency of cannabis products,5 which recent studies suggest may have occurred during the late 1990s.14 It is unclear whether increased THC content has been accompanied by any changes in CBD content. Any health effects of increased potency depend on whether users are able and willing to titrate their dose of THC, and might also vary with the experience of users. A high THC content can increase anxiety,
Conclusions
Acute adverse effects of cannabis use include anxiety and panic in naive users, and a probable increased risk of accidents if users drive while intoxicated (panel 1). Use during pregnancy could reduce birthweight, but does not seem to cause birth defects. Whether cannabis contributes to behavioural disorders in the offspring of women who smoked cannabis during pregnancy is uncertain.
Chronic cannabis use can produce a dependence syndrome in as many as one in ten users. Regular users have a
Search strategy and selection criteria
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