The Health Impact of Regular Marijuana Use
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.”