Twenty patients will be treated with creatine for 3 months (3 g daily in the first month and then 5 g daily for another 2 months) and then for 3 months with placebo. The other twenty patients will be administered placebo for 3 months and then creatine for 3 months in the same dosages and procedure. Patients' neuroleptic treatment will not be affected by study participation. Mood stabilizers, benzodiazepines and anticholinergic medications are allowed but doses will be documented throughout the study. Routine blood tests including kidney function, liver function as well as plasma creatine and creatinine will be monitored at baseline and monthly throughout the study.