OBJECTIVE: Alcohol use among college students during 21st birthday celebrations constitutes a well-known example of event-specific drinking when alcohol use is both pervasive and heavy. Less is known about how 21st birthday alcohol use compares to other birthday celebrations during young adulthood, whether similar increases occur for cannabis use on 21st birthdays, and whether the "21st birthday effect" is similar for noncollege young adults. Alcohol and cannabis use during 19th to 25th birthday celebrations were explored among college and noncollege students. METHOD: Participants were 720 young adults of ages 18 to 23 (M = 21.1, SD = 1.7) at enrollment who completed 24 monthly surveys, and 204 reported on a 21st birthday. Participants resided in a state where cannabis was legal and were asked the month following their birthday whether they engaged in alcohol and cannabis use as part of their birthday celebration. RESULTS: Multilevel models found a 21st birthday effect for alcohol use as individuals consumed over twice as many drinks on their 21st birthday than would have been expected given age trends in birthday drinking, and this effect held for college and noncollege students. A 21st birthday effect for cannabis was not found. CONCLUSIONS: 21st birthday celebrations represent a high-risk drinking event for young adults in general, and the current findings suggest event-specific prevention programs targeting all young adults turning 21 are warranted. Unlike alcohol where turning 21 is associated with socially and culturally normative use, a similar 21st birthday effect was not found for cannabis. (PsycInfo Database Record (c) 2022 APA, all rights reserved).