Introduction: The aim of this study was to test the a priori hypothesis that the increasing incidence of testis and breast cancer in adolescent and young adult (AYA) Americans correlates with their increasing cannabis use. 

Methods: The overall study design involved comparing breast and testis cancer incidence trends in jurisdictions that had or had not legalized cannabis use. Cancer incidence was assessed for the U.S. using the U.S. Surveillance, Epidemiology, and End Results (SEER) data, and for Canada, using Institute for Health Metrics and Evaluation data.

Results: In the U.S., both breast carcinoma in 20- to 34-year-old females and testis cancer in 15- to 39-year-old males had annual incidence rate increases that were highly correlated (Pearson’s r = 0.95) with the increase in the number of cannabis-legalizing jurisdictions during the period 2000–2019. Both were significantly greater during the period 2000–2019 in the SEER registries of cannabis-legalizing than non-legalizing states (Joinpoint-derived average annual percent change, AAPC1.3, p << 0.001 vs. 0.7, p << 0.001, respectively, for breast cancer, and AAPC1.2, p << 0.001 vs. no increase during the period 2000–2011 for testis cancer). During the period 2000–2019, registries in cannabis-legalizing versus non-legalizing states had a 26% versus 17% increase in breast carcinoma and 24% versus 14% increase in testis cancer. In the same age groups, Canada had a greater increase in both breast and testis cancer incidence than the U.S., and in both countries, breast and cancer trends were both correlated with the country’s cannabis use disorder prevalence by age. 

Conclusions: North America shows evidence that cannabis is a potential etiologic factor contributing to the rising incidence of breast carcinoma and testis cancer in young adults. Canada’s greater increases than in the U.S. are consistent with its earlier and broader cannabis legalization. Given the increasing use and potency of cannabis facilitated by jurisdiction legalization and expanded availability, cannabis’ potential as a cause of breast and testis cancer merits national consideration.