Understanding US adolescents' and emerging adults' overestimation of their cannabis use quantity

Cannabis is the most used federally illicit drug in the United States (U.S). Understanding how accurate adolescents and emerging adults are at estimating their cannabis use quantity is important, as this lays the groundwork for understanding and studying the outcomes associated with cannabis use. This study aimed to assess the accuracy of cannabis quantity in grams for a typical joint and identify predictors of accuracy among U.S. adolescents and emerging adults. Participants (n = 50; aged 14-21) who reported using joints in the past 30 days completed measures of cannabis use characteristics and problems, were asked to "eyeball" the amount of cannabis in their average joint using a cannabis substitute and estimated the number of grams. The cannabis substitute was then weighed in grams. Paired samples t-tests indicated that participants estimated significantly more grams in a joint (M=.82; SD = 0.46) than the actual amount of the surrogate substance (M=.47; SD = 0.21; t(49) = 6.32, p < 0.001, d = 0.89). A linear regression analysis indicated that the age of cannabis initiation (B = -0.13, p=.002) and cannabis use problems (B = -0.03, p=.009) were negatively and significantly associated with discrepancies in estimated grams. Specifically, being older when first using cannabis and reporting more cannabis use problems were linked to less overestimation of grams per joint. These findings align with previous research demonstrating a tendency to overestimate cannabis quantity and contribute novel insights by identifying predictors of estimation accuracy. This study underscores the importance of considering age and cannabis-related problems when assessing the reliability of self-reported cannabis use quantities among adolescents and emerging adults.