Reis et al. Respond

We thank Chen et al. for their letter. Chen et al. assert that lifetime use is not a “good measure” for investigating whether marijuana contributes to the development of cardiovascular disease (CVD). However, a recent report from the National Academies Committee on the Health Effects of Marijuana noted several important limitations of studies on marijuana use and CVD.¹ Two of these shortcomings are the reliance on only a single assessment of marijuana and the lack of studies on lifetime use. Our study estimated lifetime use based on information collected at up to eight assessments every two to five years over a 25-year period beginning in early adulthood, thereby addressing both limitations in the literature simultaneously. This methodology has been used in several studies published by our research group.^2–7

Chen et al. state that “77.6% of the result by Reis et al. study on lifetime use is a reflection of the effect of quitting rather than using marijuana.” We believe it is both inaccurate and misleading to use the percentage of former marijuana users in the cohort to explain why our findings mask any potential adverse effect of marijuana use on CVD. Lifetime use, by its very definition, estimates exposure over the entire course of one’s lifetime regardless of the time period in which one was exposed. Thus, lifetime use is also estimated among former marijuana users.

Chen et al. seem to have taken the sample sizes in Table 2 to calculate a crude relative risk of CVD for recent users of marijuana. However, they did not account for the large number of factors, such as tobacco cigarette smoking, that may confound this association. When adjusted for the potential confounding factors listed in the footnote to Table 2, the adjusted hazard ratio comparing recent with no recent use was 0.96 (95% confidence interval = 0.64, 1.43). The authors suggest that future studies should examine the effect of quitting on CVD risk, but these results suggest little evidence for an association of recent marijuana use with incident CVD.

We concluded that neither cumulative lifetime nor recent use of marijuana is associated with the incidence of CVD. Because we were unable to directly address the question of whether marijuana may act as a trigger of acute coronary heart disease, we further concluded that potential marijuana users, particularly those at high risk, should be counseled about this potential concern. We believe these conclusions remain a valid reflection of our findings.