Cannabis is associated with an increased risk of mental disorders, including the onset or adverse evolution of schizophrenia, bipolar disorder and depression. The aim of our study was to examine how cannabis use influences length of stay and readmissions in a unit of involuntary care.Method: All patients admitted to one secure adult psychiatry unit in France in 2016 were included (n = 370). Patients were assigned to one of two groups according to detection of urinary tetrahydrocannabinol (THC + or -). Clinical outcomes, such as length of stay and readmissions, were compared between the groups in multivariate analyses.Results: We identified 130 THC + patients and 240 THC- patients. THC + patients were often young men. In adjusted analyses, THC + status was significantly associated with one-year readmissions (OR = 2.29, p = .0082) and more prescriptions of benzodiazepines (OR = 1.93, p = .02), but not antipsychotics, at discharge.Conclusions: Cannabis users seem to have a particular profile in secure units, and are associated with specific diagnoses and treatments, and a higher risk of readmissions. Adapted management strategies might be warranted for these patients.