A recent global review of marijuana (cannabis) suggests it has been used by one in 25 adults aged 15 to 64 years. Published in the Lancet, the report focuses on nonmedical use. Its authors, led by Professor Wayne Hall of the University of Queensland, Australia, say that cannabis is the most widely used illicit drug by young people in high-income countries.

It has recently become popular on a worldwide scale, they explain. But regular use “can have adverse outcomes.” They examined those of most interest for public health – dependence, risk of vehicle crashes, bronchitis and other airway diseases, heart disease, and effects on lifestyle and mental health.

It is estimated that 166 million adults worldwide used cannabis in 2006. Use was highest in the U.S., Australia and New Zealand, followed by Europe. It typically began in teenage years and declined after obtaining full-time employment, getting married, and having children.

The active component of cannabis is tetrahydrocannabinol (THC). Short-term side effects can include anxiety, changes in appetite, panic reactions and even psychotic symptoms. About nine percent of users will become dependent, compared with 32 percent for nicotine and 15 percent for alcohol. Withdrawal may trigger insomnia and depression.

Chronic bronchitis can develop, as cannabis smoke contains many of the same carcinogens as tobacco smoke. Heavy users are at higher risk of problems with verbal learning, memory, and attention. Use is also linked to poor educational attainment, but the experts say that the cause and effect of this relationship is unclear. It may be caused by pre-existing risk factors as well as cannabis use.

Because cannabis can slow reaction time and coordination, it brings an increased risk of road accidents. Its use in pregnancy could reduce birthweight, but does not seem to cause birth defects. Cannabis users are also more likely to go on to use other illicit drugs, including heroin and cocaine.

The potential link to schizophrenia causes widespread concern. Studies suggest the risk is more than doubled for people who have tried cannabis by age 18. An analysis published in the Lancet in 2007 found a 40 percent increase in risk of “psychotic symptoms or disorders” in people who had used cannabis, with the highest risk among regular users, particularly those with a vulnerability to psychosis. For depression and suicide attempts, the evidence is less clear.

The University of Queensland experts conclude that, “The most probable adverse effects [of cannabis] include a dependence syndrome, increased risk of motor vehicle crashes, impaired respiratory function, cardiovascular disease, and adverse effects of regular use on adolescent psychosocial development and mental health.”

In a separate study, the experts take an in-depth look at the possible risk of psychosis. They say that observational studies show “consistent evidence that cannabis is associated with an increased risk of schizophrenia, and more generally, psychosis.” But there is debate about whether cannabis is a true contributing cause.

Since 2004, there has been a great deal of research carried out regarding the link. Overall, these studies suggest that the association is unlikely to be due to chance. “The evidence suggests that it is more likely that cannabis use precipitates psychosis in vulnerable persons, which is consistent with other lines of evidence suggesting that there is a complex constellation of factors leading to psychosis,” they write.

“We argue that the evidence is as good as that for many other risk factors,” they add. “Psychotic disorders are associated with substantial disability, and cannabis use is a potentially preventable exposure.”

When the Australian team investigated whether cannabis is linked to higher overall risk of death, they found “insufficient evidence, mainly due to the low number of studies.” Some studies suggest that certain health outcomes may be elevated among heavy users, yet there is a lack of long-term research that follows cannabis users into old age, when harmful effects are more likely to emerge.

Conversely, cannabis has been tried as an experimental treatment for gastrointestinal conditions such as inflammatory bowel disease. Cannabinoid receptors are located throughout the gut, involved in the regulation of food intake, nausea and inflammation. Drugs based on cannabis that act on these receptors may have therapeutic potential, scientists believe.

Cannabis preparations are also used as a remedy for chronic pain. In a 2009 review, researchers state that cannabis is “moderately efficacious for treatment of chronic pain,” but the beneficial effects “may be partially (or completely) offset by potentially serious harms.” More evidence from larger trials is needed, they conclude.