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• This is the first study to examine cannabis use changes across clinical ED groups. • A general decrease in regular cannabis use occurred between 2004 and 2014. • In 2014 a breakpoint occurred, whereafter regular cannabis use rates began to rise. • Relaxation of medical cannabis laws coincides with increasing cannabis use rates. • Comorbid psychiatric diagnoses did not increase the odds of regular cannabis use. Regular cannabis use (CU), defined as "weekly or more often", is associated with a number of negative mental health outcomes. In the last decade, Canada legalized first medical and then recreational CU. Despite higher prevalence in mental health populations, little research has documented changes in frequency of CU with progressive legalization of cannabis. This study examined rates of CU in a sample of 843 treatment-seeking patients with eating disorders (ED) in an outpatient setting between 2004 and 2020. Across ED diagnoses, segmented regression indicated a significant break-point in regular CU in 2014, commensurate with the relaxation of medical cannabis laws. Regular CU increased from 4.9 % to 23.7 % from 2014 to 2020; well above the stable 6 % found in the general population. No significant break-point was observed in either alcohol or illicit substance use over the same time period. Significant increases in regular CU were found in patients with anorexia nervosa and binge eating disorder, while regular use remained stable in patients with bulimia nervosa. Comorbid psychiatric diagnoses did not increase odds of regular CU. Findings suggest certain patient groups with mental illness may be at risk of engaging in high frequency use in the context of legislation implying medical benefits of cannabis. [ABSTRACT FROM AUTHOR]