It's time for a new discussion of Marijuana's risks

A major risk with cigarettes, heart disease, isn't clearly seen with pot smoking. Only two studies quantified the risk between marijuana use and heart attacks

The benefits and harms of medical marijuana can be debated, but more states are legalising pot, even for recreational use. A new evaluation of marijuana’s risks is overdue.

Last year, the National Academies of Sciences, Medicine and Engineering released a comprehensive report on cannabis use. At almost 400 pages long, it reviewed both potential benefits and harms. Let’s focus on the harms.

Cancer

The greatest concern with tobacco smoking is cancer, so it’s reasonable to start there with pot smoking. A 2005 systematic review in the International Journal of Cancer pooled the results of six case-control studies. No association was found between smoking marijuana and lung cancer. Another 2015 systematic review pooled nine case-control studies and could find no link to head and neck cancers.

Another meta-analysis of three case-control studies of testicular cancer found a statistically significant link between heavier pot smoking and one type of testicular cancer. But this evidence was judged to be “limited” because of limitations in the research (all of which was from the 1990s).

There’s no evidence, or not enough to say, of a link between pot use and esophageal cancer, prostate cancer, cervical cancer, non-Hodgkin’s lymphoma, penile cancer or bladder cancer. There’s also no evidence, or not enough to say, that pot has any effect on sperm or eggs that could increase the risk of cancer in any children of pot smokers.

Heart disease

Another major risk with cigarettes, heart disease, isn’t clearly seen with pot smoking. Only two studies quantified the risk between marijuana use and heart attacks. One found no relationship at all, and the other found that pot smoking may be a trigger for a heart attack in the hour after smoking. But this finding was based on nine patients, and may not be generalisable.

Lung function

It also makes sense to think about the risk of respiratory disease. In the short term after smoking pot, a 2007 systematic review found, lung function actually improved. But these benefits were completely overtaken by evidence that lung function may degrade with chronic use. Lung function, however, is a laboratory measure and not necessarily a clinical outcome, and what we really care about is lung disease. Once you control for tobacco use, the links between marijuana and chronic obstructive pulmonary disease appear minimal. Almost no evidence is available to link pot use to asthma.

Impaired driving

Driving while impaired is a major cause of injury and death in the US. Six systematic reviews were considered of fair or good quality by the national academies, and the most recent one pooled three of the others. It contained evidence from 21 studies in 13 countries representing almost 240,000 participants.

For people who reported marijuana use, or had THC detected through testing, their odds of being involved in a motor vehicle accident increased by 20 to 30 percent, the study found. This is, of course, a relative increase, and shouldn’t be confused with the overall percentage chance of getting in an accident, which is much smaller.

Regardless, driving while impaired is a terrible idea. Although we have good tests to determine if people are under the influence of alcohol, no such tests are currently available for marijuana, making enforcement more difficult.

Pregnancy effects

Babies born to women who smoke pot during pregnancy are more likely to be underweight, delivered premature and admitted to a neonatal intensive care unit, according to a 2016 systematic review. But there were no links seen for changes in birth length, head circumference or congenital malformations. There’s limited evidence for pregnancy complications for mothers, and there’s not enough evidence to comment on much else about babies and their outcomes.

Memory and concentration

There’s moderate evidence, from many studies, that learning, memory and attention can be impaired in the 24 hours after marijuana use. There’s limited evidence, however, that this translates into worse outcomes in academic achievement, employment, income or social functioning, or that these effects linger after the pot has “worn off.”

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