Background Cannabis use is a major problem in inner cities and has been causally implicated in psychosis. Very few of the available hospital-based studies of the implications of cannabis usage have involved psychiatric intensive care units (PICU); but PICU receive many of the most challenging and resource-hungry-and incompletely understood-patients in the mental health system. Aims To study the clinical impact of cannabis abuse in a PICU, and to compare the use of atypical and typical antipsychotics in this setting. Method 115 patients admitted to a PICU consented to take part in an open label naturalistic study. BPRS, TCI-240, weight, length of admission and routine bloods were evaluated in all participants. Results There was a high rate of cannabis abuse (71.3%) in the PICU population. Patients who abused cannabis spent longer in PICU because their psychoses were more severe. They were younger at first hospital admission. Cannabis also had metabolic implications, with higher blood glucose levels at admission and greater weight increase. Atypical antipsychotics were effective in treating psychosis inpatients positive to cannabis at admission. Conclusion Our findings suggest that cannabis abusers had a more severe psychotic illness, especially in schizophrenia. There are additional complications in terms of weight gain for cannabis users.