OBJECTIVE: It remains unclear whether there is an association between severity of cannabis use and psychotic symptom severity over time. Shedding light on this under-researched matter could have clinical implications for this patient group. METHODS: This was a secondary analysis of a randomized, parallel-group, superiority, assessor-blinded trial. We followed 60 patients with dually diagnosed psychosis and cannabis use disorders from the Danish CapOpus trial, which included assessments at baseline, post-treatment (6 months) and 10 months. Cannabis use was registered by self-report assisted by timeline follow-back. Psychotic symptoms were measured using the Positive and Negative Syndrome Scale (PANSS) positive, negative, and general symptoms scores. Analyses were adjusted for potential confounders. RESULTS: Patients were classified into four categories: minor use (0-30 joints at baseline and 0-9 joints at follow-up; n = 19), moderate use (0-30 joints at baseline and 10-196 joints at follow-up; n = 11), high (reducing) use (31-240 joints at baseline and 0-9 joints at follow-up; n = 9), and severe use (31-240 joints at baseline and 10-196 joints at follow-up; n = 21). Those with severe and persistent cannabis use (severe use group) had significantly higher scores, as compared to those with minor use, on the positive symptom (17.0, 95% CI [4.7-19.2] vs. 12.7, 95% CI [10.4-15.0], respectively, adjusted p < .009) and general symptom (37.4, 95% CI [34.0-40.8] vs. 29.8, 95% CI [26.3-33.3], respectively, adjusted p < .003) scales at follow-up. The severe use group had significantly higher scores, as compared to the moderate use group, on the negative symptom scale at follow-up (17.4, 95% CI [15.1-19.7] vs. 12.5, 95% CI [9.3-15.6], respectively, adjusted p < .02). On the other hand, patients in the high (reducing) use group demonstrated the greatest improvement in psychotic symptoms on all three measures. CONCLUSIONS: These findings are preliminary and more research must be done to elucidate the relationship between cannabis use and psychosis. Treatment of psychosis and comorbid cannabis use disorder could in the future incorporate treatment strategies emphasising encouragement to reduce cannabis use. CapOpus is registered at clinicaltrials.gov (NCT00484302).