There is clear evidence of illicit drug abuse in psychiatric hospitals (Deahl, 1991), and illicit drug use has been associated with relapse in people with serious mental health problems. Cannabis abuse in the general population is common and has both desirable and undesirable effects on users. However, when cannabis is used by people with serious mental health problems, a more pronounced effect may be seen. There is little evidence to suggest that cannabis 'causes' schizophrenia, but its abuse may prove to be a trigger in vulnerable individuals. Studies have demonstrated that cannabis abuse can exacerbate the symptoms of schizophrenia and may reduce the effectiveness of antipsychotic drugs. Mental health nurses need to develop interventions in order to manage illicit drug abuse effectively in both the hospital and community settings.