A 20-year medical review shows regular marijuana use won’t kill you, but it will increase your chances of developing psychotic symptoms or addiction to the drug – especially if you start smoking at a young age.

A review published Monday in the scientific journal Addiction seeks to summarize all that doctors have learned about marijuana in the last 20 years. According to the latest research, marijuana use can impair people’s ability to drive and produce a number of adverse effects in regular users, including addiction and various psychological disorders.

Regular smokers who start using the drug in their teenage years are even more at risk of developing these adverse effects, according to review author Wayne Hall.

Hall, a World Health Organization expert advisor on addiction, reviewed cannabis research since 1993 for his article titled “What has research over the past two decades revealed about the adverse health effects of recreational cannabis use?” Hall’s article defines a regular cannabis user as someone who smokes marijuana nearly every day.

According to the studies Hall reviewed, regular cannabis users face a one-in-10 chance of developing a dependency on the drug. However, that number goes up to one-in-six for users who started smoking regularly in their teenage years.

Hall says cannabis has become more potent over the years as people have turned to cannabis plants with higher levels of THC, the active ingredient in marijuana. Addiction cases have also gone up over that time. “The number of cannabis users seeking help to quit or control their cannabis use has increased during the past two decades,” Hall says in the article.

How it affects your brain

Regular marijuana users increase their chances of developing psychotic symptoms and disorders like schizophrenia, particularly if they have a family history of mental illness, Hall found. Users who start smoking in their adolescence are twice as likely to develop a disorder, he said.

For people already at risk of developing psychosis, marijuana can bring those symptoms on sooner, Hall found. The drug essentially makes it easier for psychosis to come to the surface in people who show a tendency toward it.

Hall’s review also found teenagers who use marijuana regularly are more likely to develop an intellectual impairment. Hall says according to a number of studies, regular teenaged marijuana users fare worse in school than their non-using peers. “Studies showed that the earlier the age of first cannabis use, the lower the chances of completing school and undertaking post-secondary training,” Hall’s review said.

Hall also concludes that regular cannabis use causes a person’s IQ to drop over time. “The decline in IQ was largest in those who began using cannabis in adolescence and continued near-daily use through adulthood,” he said.

Non-lethal drug still dangerous for drivers

Hall says there is almost no chance of someone dying from a marijuana overdose, since even a very heavy user cannot take in enough THC to reach lethal levels.

However, marijuana can still be deadly if a user gets behind the wheel of a car.

Hall says marijuana impairs reaction time, hand-eye coordination and cognitive abilities, making it difficult for a driver to judge and react to situations quickly. “Cannabis users who drive while intoxicated approximately double their risk of a car crash,” Hall said.

That risk is even higher when marijuana is combined with alcohol.

Pregnancy concerns

Babies whose mothers smoked marijuana while pregnant showed a number of adverse effects, according to the studies in Hall’s review.

Hall says babies born to pot-smoking mothers often have a slightly below-normal birth weight, and tend to be more easily startled. Their eyesight develops slower than it does for other babies, and later in life, they are more likely to show “delinquency and problem behaviour,” he said.

By grade school, children of marijuana-smoking mothers often fare poorly in reading and spelling tests, Hall’s review found.

Other health risks

Hall’s review of cannabis research turned up a number of other long-term health issues linked to the drug, including lung cancer, cardiovascular disease and increased risk of heart attack. “The cardiovascular risks of cannabis smoking are probably highest in older adults, but younger adults with undiagnosed cardiovascular disease may also be at risk,” Hall said.

However, Hall says it’s tougher to link marijuana to lung cancer because many marijuana smokers also smoke tobacco, which is known to cause lung cancer.

Hall says marijuana is the third-most common drug addiction in Canada, behind only alcohol and tobacco. He estimates between one and two per cent of adults per year will be affected by marijuana addiction, and up to eight per cent of adults will deal with marijuana dependency in their lifetime.