Background: With increasing use of cannabis, we need to know if cannabis use and Body Mass Index (BMI) are associated.

Methods: The Coronary Artery Risk Development in Young Adults Study followed Black and White adults over 30 years with assessments every 2 to 5 years in four centers in the USA. We assessed self-reported current and computed cumulative cannabis exposure at every visit, and studied associations with BMI, adjusted for relevant covariables in mixed longitudinal models. We also applied marginal structural models (MSM) accounting for the probability of having stopped cannabis over the last 5 years.

Results: At the Year 30 visit, 1,912 (58 %) identified as women and 1,600 (48 %) as Black, mean age was 56 (SD 2) years. While 2,849 (85 %) had ever used cannabis, 479 (14 %) currently used cannabis. Overall, participants contributed to 35,882 individual visits over 30 years. In multivariable adjusted models, mean BMI was significantly lower in daily cannabis users (26.6 kg/m², 95 %CI 26.3 to 27.0) than in participants without current use (27.7 kg/m², 95 %CI 27.5 to 27.9, p < 0.001). Cumulative cannabis use was not associated with BMI. The MSM showed no change in BMI when stopping cannabis use over a 5-year period (β=0.2 kg/m² total, 95 %CI -0.2 to 0.6).

Conclusions: Current cannabis use was associated with lower BMI, but cumulative cannabis use and cessation were not. This suggests that recreational cannabis use may not lead to clinically relevant changes in BMI and that the association between current cannabis use and lower BMI is likely due to residual confounding.