OBJECTIVES: Examine whether an overconsumption of tranquillizers exists in prison and discuss possible reasons. METHODS: Comparative study during three weeks at Geneva: prison outpatient service and Medical Policlinic (MP) of the University Hospital. RESULTS: When comparing the 113 (prison) and 151 (MP) male patients younger than 39 years, we found important differences concerning the quality and quantity of prescriptions of psychoactive drugs: ten times more prison patients than patients from the MP were treated with benzodiazepines (BZD). The differences persisted even when considering only prisoners who were not known to be street drug, alcohol or long time BZD consumers. CONCLUSIONS: The differences cannot be explained by the high percentage of drug addicts in prison. Our results suggest the importance of factors related to the prison environment.