What 20 Years of Research on Marijuana Has Taught Us

Marijuana research since 1993 has shown its use is associated with numerous adverse health effects, including a doubling of the risk of an automobile accident, an increase in the likelihood of psychotic disorders and a heightened risk for physical problems including cancer.

Marijuana, the most well-known form of the plant-based drug cannabis, is increasingly regarded as a socially acceptable, relatively harmless substance in the U.S. and certain other countries. However, research has consistently shown that marijuana users expose themselves to a range of mental/psychological and physical harms. In a study review published in October 2014 in the journal Addiction, a team of researchers from Australia and the United Kingdom compared the scientific knowledge of marijuana/cannabis-related harm in 2013 to the level of knowledge in 1993. These researchers confirmed the ability of marijuana/cannabis to significantly damage the health of its users.

Marijuana Use in the U.S.

The federal Substance Abuse and Mental Health Services Administration tracks year-to-year changes in marijuana use throughout America as part of an annual project called the National Survey on Drug Use and Health (NSDUH). The results from the most recent version of this survey, which covers the year 2013, were released to the public in September 2014. The 2013 NSDUH findings indicate that 19.8 million U.S. adults and teenagers use marijuana at least once in the typical month; this number is equal to 7.5 percent of the entire national population over the age 11. The number of monthly marijuana users rose by roughly 900,000 people between 2012 and 2013. Broadly speaking, marijuana intake is highest in individuals between the ages of 18 and 25. Preteens and teens between the ages of 12 and 17 use the drug somewhat more often than adults age 26 or older. Men and boys consume marijuana more often than women and girls.

Previously Established Harms

Over the last several decades, studies from a variety of American and international institutions have tied marijuana use to a host of mental/psychological and physical problems. For example, research indicates that the nationwide pool of marijuana/cannabis users has a roughly one in nine chance of developing a marijuana/cannabis addiction; the risks are substantially higher for both teen users of the drug and people who consume the drug on a daily or near-daily basis. In a teen user, heavy marijuana consumption can lead to disrupted brain development, an impaired ability to do such things as retain new information and think clearly, and possibly permanent declines in overall mental abilities (as measured by an IQ test) during adulthood. Regular marijuana/cannabis users are unusually likely to develop the symptoms of a diagnosable mental illness; in addition, heavy consumers of the drug may experience short-term episodes of psychosis, a debilitating combination of symptoms that partially severs the normal link to reality. Physical problems linked to marijuana/cannabis intake include higher chances of developing lung infections and other lung ailments.

Confirmed Marijuana-Related Damage

In the study review published in Addiction, researchers from King’s College London and Australia’s University of Queensland and University of New South Wales analyzed previous studies, conducted between 1993 and 2013, that focused on one or more aspects of the short- and long-term harms associated with marijuana/cannabis use. The researchers undertook their project, in part, in response to the growing popularity of the drug, especially among those individuals in the 18-to-25 age range.

After completing their review, the researchers confirmed the ability of marijuana/cannabis to produce significant short-term harms. Prominent among these harms are a roughly 100 percent increase in cannabis users’ odds of getting involved in a motor vehicle accident. The researchers note that the motor vehicle accident-related risks of marijuana/cannabis intake rise even higher in people who combine the drug with alcohol.

The researchers also confirmed a range of negative long-term impacts associated with regular marijuana/cannabis use. Prominent examples of these impacts include a roughly 17 percent chance of cannabis addiction in people who start using the drug as teenagers, a doubling of the odds that a teenager will eventually develop psychosis and/or qualify for a schizophrenia diagnosis as an adult, a higher chance that a teenager will use other illicit/illegal drugs and higher chances that a teenager will underperform academically. In addition, regular consumers of marijuana/cannabis are more likely to experience symptoms of chronic bronchitis, one of the two conditions that constitute the ultimately lethal ailment known as chronic obstructive pulmonary disorder (COPD). Finally, heart attack risks appear to increase in middle age adults who consume the drug.