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 countries (figures 1 and 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). 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. Some African countries are also thought to have high rates of cannabis use. Because of their large popu lations, 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).
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 offtowards the end of the decade. A similar rise in its use in the early 1990s, followed by decline or stabilisation in re cent years, has been reported in Australia and western Europe.
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. Cannabis use in the USA typically begins in the middle to late teenage years and peaks in the early and middle 20s. Use declines steeply after young people enter full-time employment, marry, and have children.
No reliable information exists about the concentration of Δ-9-tetrahydrocannabinol and other cannabinoids (eg, cannabidiol) in commonly used cannabis products. In epidemiological studies, heavy or regular cannabis use is usually defined as every day or almost every day use. This pattern, when continued over years, predicts an increased risk of some adverse health effects.This review summarizes the most probable adverse health effects of cannabis use during the years since the last review in 1997 by WHO.
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 settingie, the user's expectations, attitudes towards the effects of cannabis, the mood state, and the social setting in which it is used. 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 such as eating, watching films, listening to music, and engaging in sex.8 When used in a social context, the so-called high could be accompanied by infectious laughter, talkativeness, and increased sociability. These effects typically occur 30 min after smoking and last for 1-2 h
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