About one in 25 people ages 15 to 64 have smoked marijuana (aka cannabis, pot) worldwide, according to Wayne Hall, PhD, professor of public health policy in the School of Population Health, University of Queensland, Brisbane, Australia, and the co-author of the review published in The Lancet.
A U.S. expert on the health effects of marijuana says the public health impact of marijuana is "miniscule" compared to the effects of alcohol. The Australian experts acknowledge that the public health burden of marijuana is probably modest compared to that of alcohol, tobacco, and other illicit drugs.
Marijuana Heaalth Study
Hall searched the medical literature for the past 10 years, looking for studies that focused on adverse health effects of marijuana. According to Hall, marijuana use peaked in the U.S. for young adults in 1979, then decreased until the early 1990s, when it again increased before leveling off toward the end of the 1990s.
But marijuana use varies globally, Hall found. "Trends in use over the past decade have varied between regions and countries," Hall says. Use has stabilized or fallen in many developed countries while increasing in some developing countries, he says.
"The main points of the paper are that evidence has strengthened for the existence of a dependence syndrome, an increased risk of motor vehicle accidents if users drive while intoxicated, impaired respiratory function in daily smokers, psychoses in young people who begin in their mid-teens and use daily or near daily, and poorer psychosocial outcomes in adolescents who initiate early and become regular users," according to Hall.
The researchers estimate that about one in ten people who ever use marijuana will become dependent on marijuana.Avoiding Marijuana Health Risks.
Marijuana Use and Health
In the medical literature review, Hall found:
- About 9% of those who ever use marijuana become dependent. But the risk rises to one in six if use begins in teen years. About 10% of ever-users become daily users, and 20% or 30% become weekly users.
- Regular and heavy pot use has been linked with problems in memory, attention, and verbal learning, but researchers aren't certain whether those changes are transient and disappear once marijuana use is stopped.
- Regular marijuana users, especially if they started at a young age, are more likely to later use other drugs such as heroin and cocaine, according to some studies.
- Early marijuana use, before age 15, has been linked with school dropouts. But researchers are unsure exactly how to explain the association. One possibility: poor school performance can trigger the pot use, which in turn makes school performance even worse.
- The use of marijuana by age 18 is linked with more than a doubling of risk for a later diagnosis of schizophrenia, according to a large Swedish study.
- Marijuana use increases the heart rate, and adults with existing heart disease may be at higher risk of a heart attack after pot use, some research suggests.
- Driving after smoking marijuana increases the risk of a motor vehicle accident by two to three times, research suggests, while driving after drinking alcohol increases accident risk six to 15 times.
- Symptoms of chronic bronchitis were more commonly reported among regular marijuana smokers than nonsmokers. Marijuana smokers reported wheezing, chronic cough, and production of sputum.
- Studies looking at a possible link between an increased risk of cancers in the upper respiratory tract and marijuana use have produced mixed findings, with some finding a link and others not.
- Smoking pot during pregnancy has been linked with underweight babies, but there is little evidence it's linked to birth defects.
Hall notes that some people are at greater risk for adverse health effects from smoking marijuana, especially "young persons under the age of 18 and ... persons with any disease or condition (for example, cardiovascular disease, respiratory disease, mental illness, pregnancy, or other types of substance abuse) which increases their vulnerability to adverse effects from marijuana use."
Hall, W. The Lancet, Oct. 17, 2009: vol 374: pp1383-1391.
Wayne Hall, PhD, professor of public health policy, School of Population Health, University of Queensland, Brisbane, Australia.