Background: Despite decades of campaigns aimed at reducing tobacco/nicotine (T/N) use and the development of many different T/N reduction and cessation strategies, the impacts on international public health remain significant. Some studies have found an association between medical and non–medical cannabis use and T/N use, although the evidence on whether cannabis/cannabinoids increase or decrease the odds of reducing or ceasing T/N use remain contradictory. This paper explores the self-reported use of cannabis and associated changes in T/N use among a Canadian medical cannabis patient population. Methods: This study examines the impact of medical cannabis on T/N use by comparing self-reported patterns of use before and after the initiation of medical cannabis. Participants completed an online cross-sectional survey examining demographics, patterns of medical cannabis use, and the impact of medical cannabis on the use of T/N and other substances. The survey also included novel measures examining whether patients intended to use medical cannabis to reduce T/N use or had experience with other pharmacological or psychobehavioral T/N cessation strategies. We conducted a series of descriptive analyses and univariate and multivariate logistic regressions to explore the potential association between primary variables of interest and T/N reduction and cessation. Results: In total, the study recruited 2102 individuals, of whom 650 were current or former T/N users. Following initiation of medical cannabis use 320 (49%) T/N users self-reported reductions in use, with 160 (24.6%) reporting no T/N use in the 30 days prior to the survey. Odds of T/N cessation were greater among those who were age 55 or older (Adjusted Odds Ratio [AOR] = 2.56, 95% Confidence Interval [CI] 1.53–4.26), or those who reported >25 T/N uses per day in the pre-period (AOR = 2.11, 95% CI 1.14–3.92). Specific intent to use medical cannabis to quit resulted in significantly greater odds of reducing T/N use (AOR = 2.79, 95% CI 1.49–5.22); however, involvement with traditional T/N cessation treatments (pharmacological or psychobehavioral) was negatively associated with T/N cessation (AOR 0.39, 95% CI 0.18–0.86). Conclusions: Results from this retrospective survey of medical cannabis users suggest that initiation of medical cannabis use was associated with self-reported reductions and/or cessation of T/N use in nearly half of study participants. In light of the significant morbidity, mortality, and health care costs related to T/N dependence, future research should further evaluate the potential of cannabis-based treatments to support efforts to reduce or cease T/N use.