The Bi-Directional Influence of Nicotine and Marijuana Use Across the Life Course: Implications for Prevention in the Context of Legalized Marijuana Use

Preventing the onset and use of nicotine remains a top priority in drug use prevention. The Gateway Hypothesis suggests that a focus on disrupting the onset of nicotine will also decrease involvement in other drugs like marijuana. Recently it has been suggested that other drugs, such as marijuana, may increase the use of nicotine. Given the changing landscape of marijuana legalization in the U.S. it will be important to understand the directional influence of nicotine and marijuana use over time. Using prospective data beginning in childhood (age 11) and extending into adulthood (age 33), this study investigated the bidirectional influence of nicotine and marijuana use across the life course to see if nicotine use predicted later marijuana use or if marijuana use predicted later nicotine use and if this relationship differed by developmental period.

Data were from the Seattle Social Development Project (SSDP), a thirty year longitudinal study that began in 1985 and followed 808 youth from ages 10-39. Participants included the 689 participants (85% of the original sample) who had ever used either nicotine or marijuana before age 33. Cross-lagged models used data from ages 13 to 33 to test whether smoking frequency at one time point predicted marijuana frequency at the following time point, or the reverse. Smoking before the age of 12, ethnicity, gender and socio-economic status were included as covariates in the model.

Results supported both the Gateway Hypothesis directionality (nicotine -> marijuana) and the Reverse Gateway Hypothesis (marijuana - > nicotine). During adolescence, smoking before age 12 predicted marijuana use at age 13 (b=.32, p<.001) and smoking at age 14 predicted marijuana use at age 15 (b=.16, p<.05).Marijuana use did not predict smoking at any time in adolescence. Thus, results supported the Gateway Hypothesis. In the transition from young adulthood to adulthood, the Reverse Gateway Hypothesis was supported. Marijuana use at age 24 predicted cigarette use at age 27 (b=.11, p<.01), but not the reverse.

Results suggest that nicotine and marijuana use are reciprocally related and that the direction of influence differs developmentally. The finding that smoking lead to increased marijuana use in adolescence suggests current prevention strategies targeting early nicotine onset and use may also reduce marijuana use. The finding that marijuana use in young adulthood lead to increased smoking in adulthood points to an important area for novel  prevention programming and raises concerns that legalization of adult marijuana use may have unintended negative effects.