["Addiction, EarlyView. ", "\nAbstract\n\nBackground and aims\nCo‐use of cannabis and e‐cigarettes for nicotine is increasing among younger adults in the United States (US). This study aimed to determine (1) the patterns of cannabis and e‐cigarette use and co‐use over time and (2) the trajectories of e‐cigarette and cannabis dependence symptoms across latent classes representing different patterns of use and co‐use.\n\n\nDesign\nProspective cohort study, with data collected every six months at six time points.\n\n\nSetting\nHawaii, USA.\n\n\nParticipants\nN = 1305 individuals [mean age = 25.0 (standard deviation = 2.2); 52% women], who reported past‐30‐day cannabis and/or e‐cigarette use at any of the six data collection time points between March 2021 and February 2024.\n\n\nMeasurements\nFor the repeated measures latent class analysis (RMLCA): past‐30‐day cannabis use, e‐cigarette use and co‐use (any past‐30‐day use of both cannabis and e‐cigarettes for nicotine). RMLCA covariates: demographic variables and cigarette smoking. For the latent growth curve analyses: e‐cigarette dependence [Penn State E‐Cigarette Dependence Index (PSEDI), range: 0–20] and cannabis dependence symptoms [Cannabis Abuse Screening Test (CAST), range: 0–24].\n\n\nFindings\nFour latent classes were identified: individuals who, across time points, consistently showed (1) low probabilities (≤ 0.17) of using e‐cigarettes or cannabis, including co‐use (44% of the sample); and (2) high probabilities (> 0.50) of primarily e‐cigarettes‐only use (18%); (3) primarily cannabis‐only use (20%); and (4) primarily co‐use (18%). For co‐use class, the average trajectory for cannabis dependence remained flat [mean linear slope (LS) = 0.01 (standard error [SE] = 0.03), P > 0.05] whereas that for e‐cigarette dependence increased over time [LS = 0.15 (0.03), P < 0.0001] before showing a slight decline [mean quadratic slope (QS) = −0.02 (0.007), P < 0.01]. For e‐cigarette‐only use class, e‐cigarette dependence increased over time [LS = 0.91 (0.05), P < 0.0001], before showing a slight decline [QS = −0.02 (0.006), P < 0.0001]. The levels of e‐cigarette dependence were higher for e‐cigarette‐only class than the co‐use class across all time points. For cannabis‐only class, the average trajectory of cannabis dependence symptoms did not change over time [LS = 0.03 (0.03), P > 0.05]. The levels of cannabis dependence symptoms were higher for the co‐use class than the cannabis‐only class across all time points.\n\n\nConclusions\nAmong young adults who reported past‐30‐day cannabis and/or e‐cigarette use, on average, e‐cigarette dependence increased over time for both e‐cigarette‐only and co‐use classes before showing slight declines. Compared with the co‐use class, the e‐cigarette‐only class tended to show higher levels of e‐cigarette dependence across all time points. Cannabis use dependence did not change statistically significantly for either co‐use or cannabis‐only classes.\n\n"]