Marijuana is a drug of many faces—used for recreational or medical purposes, illegally or legally, depending on geography and circumstance. The current landscape of marijuana legalization in the United States is highly dynamic and controversial and will continue to grow more so as patients, healthcare providers, and the general public ask more questions about just how harmful the substance is.
Unfortunately, high-quality studies in this arena are lacking. Because recreational marijuana remains illegal in all but 2 states (Colorado and Washington), well-controlled, unbiased studies on the long-term effects of marijuana are impossible to conduct for the time being. As such, criticism and skepticism regarding the current evidence runs high, and future legislation on recreational marijuana will depend on how these 2 states fare. Last Sunday, an article in the New York Times highlighted some recent cautionary tales seen in Colorado since the legalization of recreational marijuana there in January, including some particularly alarming incidents that occurred after ingesting edible versions of the drug. However, comprehensive health statistics on adverse outcomes will take years to collect and analyze.
In the meantime, a review article published yesterday in the New England Journal of Medicine by authors from the National Institutes of Health’s National Institute on Drug Abuse summarizes the current evidence available on the adverse health effects of marijuana use. In the article, the authors discuss several of the most commonly studied and reported effects of marijuana use from studies on both animals and humans and present their take on how “real” these adverse effects are by dividing them into categories of high, medium, and low levels of confidence in the quality of the evidence.
Adverse health effects that the authors attribute to marijuana use with high confidence include
- Addiction, with an estimated 9% of those who experiment with marijuana eventually becoming addicted. This number increases to 16% among those who start using marijuana as teenagers. The clinical observation of a cannabis withdrawal syndrome upon cessation also supports the addictive nature of the chemicals in marijuana.
- Diminished lifetime achievement, which includes poorer academic performance in school and an increased risk of dropping out of school for children and adolescents
- Increased risk of motor vehicle accidents due to impaired driving ability
- Increased symptoms of chronic bronchitis
Adverse health effects that the authors attribute to marijuana use with medium confidence include
- Abnormal brain development, which can occur if marijuana is used regularly through childhood and adolescence, until the age of approximately 21 years. Although there is ample evidence suggesting that adults who used marijuana in their youth have impaired brain connectivity, it is difficult to establish a cause-effect relationship (as opposed to simply an association) among these individuals. The basis for this concern over brain development largely comes from animal studies that provide more compelling evidence for causality.
- Gateway to use of other drugs, which follows as a result of the abnormal brain development described above. It is possible that marijuana use during brain development alters the brain’s natural reward system and makes one more prone to addictive behaviors.
- Mental illness, including depression, anxiety, and psychosis. Although studies have shown association between these conditions and marijuana use, causality has not been established.
The authors note low confidence in evidence suggesting a link between marijuana use and lung cancer.
Details on the literature search (for example, the numbers of studies reviewed and the dates of those studies) is not provided, nor is information about how the authors developed the rating system for the high, medium, and low levels of confidence.
Medical marijuana, on the other hand, has been more widely studied (with both observational studies and randomized trials), and currently medical marijuana is legal in 22 states and in Washington, DC. Medical marijuana is used to treat certain medical conditions such as chronic pain, nausea, and anorexia (“wasting”) in patients with end-stage AIDS. Still, evidence on its effectiveness has been varied and not definitive, and accidental ingestion and poisoning among children remains ongoing concern. The use of medical marijuana in Colorado is discussed in a recent JAMA audio interview with Larry Wolk, MD, MSPH, executive director and chief medical officer for the Colorado Department of Public Health and Environment.