Post-traumatic stress disorder (PTSD) is a mental illness that commonly affects military and police service veterans after experiencing traumatic events throughout their service. Alcohol and tobacco are often overused by this population to help relieve the symptoms of PTSD. The objective of this paper is to examine if alcohol and tobacco use in military and police service veterans with PTSD changed after using medical cannabis for PTSD symptom management. A retrospective chart review was conducted to analyse information about alcohol, tobacco, and medical cannabis use, as well as previously attempted PTSD treatment methods. 101 patients (average age 43 years, 96.0% male, 60.4% married, and 81.2% with children) who visited a single treatment center between January 2014 and April 2016 were included in this study. The most common treatments patients tried prior to using medical cannabis included self-treatment with non-medical cannabis (87.8%), medication for depression (86.6%) and anxiety (85.4%), and psychotherapy (82.9%). At baseline, 81.2% consumed alcohol (average 8.1 drinks/week) and 84.2% smoked tobacco cigarettes (average 2.7 packs/week). At follow-up, 67.3% consumed alcohol (average 5.5 drinks/week) and 67.3% smoked tobacco cigarettes (average 2.5 packs/week). The use of medical cannabis was correlated with a reduction in alcohol and tobacco use. However, statistical significance was not reached (p = 0.11 and p = 0.65, respectively). Medical cannabis has the potential to reduce alcohol or tobacco use in PTSD patients. Further investigation is required to understand how medical cannabis can alleviate PTSD-related symptoms, and to identify its impact on other lifestyle factors, such as tobacco and alcohol consumption.