Aims A growing literature has documented the substantial prevalence of and putative mechanisms underlying co-occurring (i.e., concurrent or simultaneous) cannabis and tobacco use. Greater understanding of the clinical correlates of co-occurring cannabis and tobacco use may suggest how intervention strategies may be refined to improve cessation outcomes and decrease the public health burden associated with cannabis and tobacco use. Methods A systematic review of the literature on clinical diagnoses, psychosocial problems, and outcomes associated with co-occurring cannabis and tobacco use. Twenty-eight studies compared clinical correlates in co-occurring cannabis and tobacco users vs. cannabis or tobacco only users. These included studies of treatment-seekers in clinical trials and non-treatment-seekers in cross-sectional or longitudinal epidemiological or non-population-based surveys. Results Sixteen studies examined clinical diagnoses, four studies examined psychosocial problems, and 11 studies examined cessation outcomes in co-occurring cannabis and tobacco users (several studies examined multiple clinical correlates). Relative to cannabis use only, co-occurring cannabis and tobacco use was associated with a greater likelihood of cannabis use disorders, more psychosocial problems, and poorer cannabis cessation outcomes. Relative to tobacco use only, co-occurring use did not appear to be consistently associated with a greater likelihood of tobacco use disorders, more psychosocial problems, nor poorer tobacco cessation outcomes. Conclusions Cannabis users who also smoke tobacco are more dependent on cannabis, have more psychosocial problems, and have poorer cessation outcomes than those who use cannabis but not tobacco. The converse does not appear to be the case.

Most clinical trials use 6 mo or 1 yr follow-ups as proxies for life-time smoking cessation. Retrospective studies have estimated 2-15% of smokers relapse each year after the first 1 year of abstinence, but these have methodological problems such as memory bias. We searched for prospective studies of adult quitters that reported the number of participants abstinent at 1 yr followup and who remained abstinent at ≥ 2 year follow-ups. We included studies that reported the percent who remained lapse-free, did not continue treatment after 1 yr, and had ≤ 10% lost to follow-up. We did not locate any population-based studies but did locate eight randomized, controlled trials, all testing nicotine medications. After deleting one trial with outlier results, a meta-analysis estimated the annual incidence of relapse after 1 yr to be 10%; however, the small sample sizes resulted in a wide 95% confidence interval (5-17%) suggesting this estimate is not very accurate. We conclude a non-significant amount of relapse occurs after 1 yr. Better quantification of this relapse rate is important to improve estimates of lifelong abstinence and reductions in morbidity and mortality from smoking cessation.

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