Marijuana and your health: What 20 years of research reveals

People who drive under the influence of marijuana double their risk of being in a car crash, and about one in 10 daily marijuana users becomes dependent on the drug, according to a new review.

Marijuana use has become increasingly prevalent over the years, and the review of marijuana studies summarizes what researchers have learned about the drug's effects on human health and general well-being over the past two decades.

In the review, author Wayne Hall, a professor and director of the Center for Youth Substance Abuse Research at the University of Queensland in Australia, examined scientific evidence on marijuana's health effects between 1993 and 2013.

He found that adolescents who use cannabis regularly are about twice as likely as their nonuser peers to drop out of school, as well as experience cognitive impairment and psychoses as adults. Moreover, studies have also linked regular cannabis use in adolescence with the use of other illicit drugs, according to the review, published today (Oct. 6) in the journal Addiction.

Researchers in the studies still debated whether regular marijuana use might actually lead to the use of other drugs, Hall wrote in the study. However, he pointed to longer-term studies and studies of twins in which one used marijuana and the other did not as particularly strong evidence that regular cannabis use may lead to the use of other illicit drugs. [Marijuana vs. Alcohol: Which Is Worse for Your Health?]

The risk of a person suffering a fatal overdose from marijuana is "extremely small," and there are no reports of fatal overdoses in the scientific literature, according to the review. However, there have been case reports of deaths from heart problems in seemingly otherwise healthy young men after they smoked marijuana, the report said.

"The perception that cannabis is a safe drug is a mistaken reaction to a past history of exaggeration of its health risks," Hall told Live Science.

However, he added that marijuana "is not as harmful as other illicit drugs such as amphetamine, cocaine and heroin, with which it is classified under the law in many countries, including the USA."

The risks of using marijuana

Marijuana use carries some of the same risks as alcohol use, such as an increased risk of accidents, dependence and psychosis, he said.

It's likely that middle-age people who smoke marijuana regularly are at an increased risk of experiencing a heart attack, according to the report. However, the drug's "effects on respiratory function and respiratory cancer remain unclear, because most cannabis smokers have smoked or still smoke tobacco," Hall wrote in the review.

Regular cannabis users also double their risk of experiencing psychotic symptoms and disorders such as disordered thinking, hallucinations and delusions — from about seven in 1,000 cases among nonusers to 14 in 1,000 among regular marijuana users, the review said. And, in a study of more than 50,000 young men in Sweden, those who had used marijuana 10 or more times by age 18 were about two times more likely to be diagnosed with schizophrenia within the next 15 years than those who had not used the drug.

Critics argue that other variables besides marijuana use may be at work in the increased risk of mental health problems, and that it's possible that people with mental health problems are more likely to use marijuana to begin with, Hall wrote in the review.

However, other studies have since attempted to sort out the findings, he wrote, citing a 27-year follow-up of the Swedish cohort, in which researchers found "a dose–response relationship between frequency of cannabis use at age 18 and risk of schizophrenia during the whole follow-up period."

In the same study, the investigators estimated that 13 percent of schizophrenia cases diagnosed in the study "could be averted if all cannabis use had been prevented in the cohort," Hall reported.

As for the effects of cannabis use in pregnant women, the drug may slightly reduce the birth weight of the baby, according to the review.

More THC?

The effects of euphoria that cannabis users seek from the drug come primarily from its psychoactive ingredient, called delta-9-tetrahydrocannabinol, better known as THC, Hall wrote in the review. During the past 30 years, the THC content of marijuana in the United States has jumped from less than 2 percent in 1980 to 8.5 percent in 2006.

The THC content of the drug has also likely increased in other developed countries, Hall wrote in the report.

It is not clear, however, whether increased THC content may have an effect on users' health, the report said. [The Drug Talk: 7 New Tips for Today's Parents]

Some argue that there would be no increase in harm, if users adjusted their doses of the drug and used less of the more potent cannabis products to get the same psychological effects they seek, Hall said.

However, "the limited evidence suggests that users do not completely adjust dose for potency, and so probably get larger doses of THC than used to be the case," Hall said.

Studies on the use of alcohol — and, to a lesser extent, other drugs such as opioids — have also shown that more potent forms of these substances increase users' level of intoxication, as well as their risk of accidents and developing dependence, he added.