The review article by investigators from NIDA which focuses on recreational marijuana use shows that during intoxication, marijuana can interfere with memory, perception of time, and motor function, which can lead to serious consequences, including motor vehicle crashes. In addition, repeated use during adolescence can result in long-term brain function changes.
Noting that legal drugs, including alcohol and tobacco, account for "the greatest burden of disease" because of their widespread exposure, the authors stated that “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".
Highlights of the review of findings for recreational use are noted below:
- Addiction to marijuana is a real phenomenon. Cannabis withdrawal syndrome is characterized by irritability, insomnia, dysphoria, and anxiety. Approximately 9% of individuals who experiment with marijuana will become addicted. However, this rate increases to 17% among people who began using marijuana during adolescence, and up to half of individuals who smoke marijuana daily are addicted.
- Adults who smoked marijuana during adolescence have objective evidence of impaired neural connectivity in multiple centers within the brain. There is an association between frequent marijuana use in adolescence and lower scores on intelligence testing during adulthood.
- Marijuana use during early adolescence is associated with worse school performance and a higher risk of dropping out of school.
- Solid epidemiologic evidence exists that marijuana acts as a gateway drug to the use of other, more harmful illicit drugs. Animal models suggest that THC, the main active ingredient of marijuana, can prime the brain for enhanced responses to other drugs.
- Marijuana use is associated with higher frequencies of anxiety and depression, although no causative link has been established between marijuana use and these disorders. Marijuana may also promote a higher risk for psychosis, particularly among individuals with a preexisting genetic vulnerability.
- Heavy marijuana use has been associated with unemployment, criminal behavior, lower income, and reduced satisfaction with life.
- Marijuana is the most commonly cited illicit drug in promoting motor vehicle crashes. A meta-analysis found that using marijuana increased the risk for a motor vehicle crash approximately twofold. Other research has found that the risk for motor vehicle crashes after the use of marijuana is similar to that of persons beyond the usual legal limit of blood alcohol while driving.
- The risk for cancer associated with marijuana is unclear, but it does not appear as dangerous as tobacco. Heavy marijuana smokers may experience chronic bronchitis, but lower levels of marijuana use have a negligible effect on the risk for pulmonary disease.
The Schatman article focusing on medical marijuana and draws several conclusions, including the following:
- Because of the rise of THC concentrations, “medical” marijuana is rarely good medicine and there are a number of dangers associated with the use of whole-plant marijuana, whether used for recreational or for supposedly medical purposes.
- Though studies related to the potential use of cannabidiol (CBD), are in their infancy, this component of marijuana shows some promise for the treatment of several difficult-to-treat conditions (without the harmful effects of THC). Notably, CBD has been found to mitigate the euphoric (“high”) effects of marijuana use.