Literature Review

NIH Study Details Adverse Health Effects of Marijuana Use



As the push for legalization of recreational and medical use of marijuana continues to gain momentum, Nora Volkow, MD, and colleagues urged caution about the potential for increased adverse health consequences resulting from more widespread use of the drug, in a report published June 5 in the New England Journal of Medicine.

“The popular notion seems to be that marijuana is a harmless pleasure, access to which should not be regulated or considered illegal,” stated Dr. Volkow, Director of the National Institute on Drug Abuse in Bethesda, Maryland. “Currently, marijuana is the most commonly used ‘illicit’ drug in the United States, with about 12% of people 12 years of age or older reporting use in the past year and particularly high rates of use among young people.”

Evidence “clearly indicates” that long-term marijuana use can lead to addiction, according to Dr. Volkow. The 2012 National Survey on Drug Use and Health, for example, found that an estimated 2.7 million people age 12 and older met the DSM-IV criteria for dependence on marijuana. Adolescents are particularly vulnerable to long-term adverse effects of marijuana use, she said, likely because the brain undergoes active development until about age 21.

“The negative effect of marijuana use on the functional connectivity of the brain is particularly prominent if use starts in adolescence or young adulthood, which may help to explain the finding of an association between frequent use of marijuana from adolescence into adulthood and significant declines in IQ,” stated Dr. Volkow.

In addition to marijuana’s possible role as a gateway drug, Dr. Volkow noted that regular marijuana use has been associated with an increased risk of anxiety and depression, as well as with a negative effect on school performance. Marijuana use has also been linked with a higher risk of motor vehicle accidents; a meta-analysis found that the overall risk of involvement in an accident increases by a factor of about 2 when a person drives soon after using marijuana.

Marijuana smoking has also been associated with inflammation of the large airways, increased airway resistance, and lung hyperinflation, “associations that are consistent with the fact that regular marijuana smokers are more likely to report symptoms of chronic bronchitis than are nonsmokers,” commented Dr. Volkow.

Some studies have shown beneficial effects of marijuana use for conditions such as multiple sclerosis, epilepsy, and chronic pain. However, Dr. Volkow emphasized the need to take advantage of those potential medical benefits “without exposing people who are sick to [marijuana’s] intrinsic risks.” Some physicians, she commented, “continue to prescribe marijuana for medicinal purposes despite limited evidence of a benefit. This practice raises particular concerns with regard to long-term use by vulnerable populations.”

Overall, marijuana use has been associated with substantial adverse effects, “some of which have been determined with a high level of confidence,” wrote Dr. Volkow. “Marijuana, like other drugs of abuse, can result in addiction…. Repeated marijuana use during adolescence may result in long-lasting changes in brain function that can jeopardize educational, professional, and social achievements.

“However, the effects of a drug (legal or illegal) on individual health are determined not only by its pharmacologic properties, but also by its availability and social acceptability,” Dr. Volkow continued. “In this respect, legal drugs (alcohol and tobacco) offer a sobering perspective, accounting for the greatest burden of disease associated with drugs not because they are more dangerous than illegal drugs but because their legal status allows for more widespread exposure. As policy shifts toward legalization of marijuana, it is reasonable and probably prudent to hypothesize that its use will increase and that, by extension, so will the number of persons for whom there will be negative health consequences.”

Colby Stong

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