Methods: Participants were 5,445 students (64% female; 36% white; mean age=21) from 24 2-year and 4-year colleges in the southwestern United States participating in a six-wave longitudinal study who completed at least one survey in waves 1 (fall of 2014) through 6 (spring of 2017). Current marijuana use was assessed with a single question at each wave. “During the past 30 days, how many occasions, or times, if any, have you used marijuana? (Other names for marijuana are pot and weed)” Responses were dichotomized with “0 times” (0) and “1-2 times” (1). Current e-cigarette use was assessed with a single question at each wave. “During the past 30 days, have you used any ENDS product (i.e. an e-cigarette, vape pen, e-hookah, or mod), even one or two puffs, as intended (i.e. with nicotine cartridges and/or e-liquid/e-juice)?” Response options were “Yes” (1) or “No” (0). Multilevel mixed-effects cross-lagged path analysis, with school as the random effect, was used, controlling for wave one current tobacco use (excluding e-cigarettes), school type (four vs two year), sex, age, race/ethnicity.
Results: The cross-lagged model fit the data well, while the chi-square was significant (p<.05), the CFI was .99, the TLI was .98, and the RMSEA was .02. Current e-cigarette and marijuana stability paths were strong and significant across all waves (β ranges=.74-.83 and .82-.88, respectively). Above and beyond the stability paths, current marijuana use predicted e-cigarette use at each subsequent wave between all six waves (p<.001; β ranges=.07-.19). Current e-cigarette use at waves one and two did not predict current marijuana use at waves two and three (p>.05; β =.02 and -.01). However, current e-cigarette use at waves three, four, and five predicted current marijuana use at subsequent waves (p<.001; β ranges=.08-.16).
Conclusion: Findings extend previous research by showing a bi-directional association between e-cigarette and marijuana use over the course of 2.5 years while controlling for tobacco use. These results may reflect a general openness among young adults who use marijuana to also use e-cigarettes with nicotine and vice versa that may implicate psychosocial factors associated with both substances. Further research is needed to examine if the bi-directional association is driven by physiological effects, social influences, marketing exposure, or psychological factors.