Abstract: "Understanding similarities, differences, and associations between reasons people vape nicotine and cannabis may be important for identifying underlying contributors to their co-use...Cannabis vaping and smoking exhibited similar mean ratings for user experience and product/substance-related reasons for use. Mean ratings for reasons related to product utility were similar for cannabis vaping and nicotine vaping. Mean ratings for utility-related reasons for use were higher for cannabis vaping than cannabis smoking (mean (SD), 3.6 (± 1.0) vs. 2.6 (± 1.2), p < 0.0001). On average, harm reduction-related reasons for use were rated higher for nicotine vaping than cannabis vaping (2.4 (± 1.6) vs. 1.8 (± 1.4), p < 0.0001). Regression models showed higher average ratings for utility-related (b = 0.32; 95% CI, 0.03-0.60) and harm reduction-related (b = 0.21; 95% CI, 0.04-0.37) reasons for nicotine vaping were associated with more frequent nicotine vaping (both p < 0.05). Higher average ratings for instrumentality-related reasons for co-use corresponded with more frequent monthly nicotine vaping (b = 0.26; 95% CI, 0.08-0.44) and higher odds of ever chasing cannabis with nicotine (aOR, 3.06; 95% CI, 1.29-7.30)...Vaping serves purposes that differ by substance; nicotine vaping was more closely related to reducing tobacco smoking-related harms, and cannabis vaping was more closely related to circumventing social problems posed by cannabis smoking. Lifetime sequential co-use practices and more frequent nicotine vaping were associated with enhancing the intoxicating effects of cannabis. While replication of these findings using non convenience-based sampling approaches is warranted, results underscore the need to consider shared and unique aspects of nicotine and cannabis vaping, as well as cross-substance interactions between nicotine and cannabis".