BACKGROUND: Co-use of cannabis and alcohol is associated with a higher prevalence of health-related, psychiatric, and psychosocial impairments. The aims of this study were to 1) examine the association between cannabis and alcohol co-use and subjective effects, 2) determine whether the time between cannabis and alcohol use is associated with subjective effects, and 3) examine whether subjective responses to co-use are associated with hazardous cannabis use over time. METHODS: Young adults who co-used cannabis and alcohol (N = 155, 55.5 % women, Mean age=21 yo) completed baseline, six-month, and twelve-month assessments of hazardous cannabis use. Co-use and subjective effects were assessed across 21 days of EMA at baseline. FINDINGS: Co-use was associated with greater momentary subjective intoxication, compared to cannabis- and alcohol-only, and stimulation and bad effects, compared to cannabis-only. Subjective intoxication was significantly higher for co-use within 0-90minutes compared to cannabis-only. Subjective intoxication was significantly higher for co-use within 0-120minutes compared to alcohol-only. Co-use within 0-30minutes was associated with significantly greater stimulation and bad effects, compared to cannabis-only use moments. The extent to which an individual becomes more intoxicated when co-using the greater the risk the individual had for increasing hazardous cannabis use six and twelve months later. CONCLUSION: Findings indicate that co-use is associated with increased intoxication, compared to cannabis or alcohol use alone, and increased stimulation and bad effects, compared to cannabis use alone. Subjective responses to co-use in a naturalistic setting are important to consider when evaluating long-term risk of cannabis use among young adults.