Objective: We examined whether medical cannabis (MC) use was associated with change in ADHD symptoms across time in a sample of adults with chronic pain. Methods: We conducted a longitudinal cohort study (N = 223) across 12 months, assessing ADHD symptoms and MC use. We used mixed-effects linear regression to test whether MC use (vs. no use) and high THC (vs. low THC) MC was associated with change in ADHD symptoms from baseline to follow-up at quarterly intervals. We stratified by baseline ADHD symptoms and pain catastrophizing. Results: MC use was not associated with change in ADHD symptoms in the full sample or those with moderate/severe ADHD symptoms. Among those with minor/no baseline ADHD symptoms, high-THC MC (vs. low-THC) was associated with a decrease in ADHD symptoms. Conclusions: The null findings among participants with moderate/severe ADHD symptoms suggests that cannabis is unlikely to be a good treatment for ADHD. The preliminary findings in those with mild ADHD symptoms should be interpreted with caution given the small sample, the modest result, and all participants had chronic pain. These results may temper enthusiasm for MC as a treatment for ADHD, but further studies in larger and more generalizable samples may be justified.