The link between marijuana, cancer, heart disease and lung function is weak at best

See It’s Time for a New Discussion of Marijuana’s Risks: You may reasonably decide the benefits outweigh the harms, but you should know about those potential harms by Aaron E. Carroll, a professor of pediatrics at Indiana University School of Medicine. Excerpt:

"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. (Using marijuana while pregnant does pose other risks, as discussed below.)

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 generalizable.

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."