Methods for treating obsessive compulsive disorder are described. In one method, a serotonin reuptake inhibitor (SRI) and ondansetron or a pharmaceutically acceptable salt thereof is administered to a patient suffering from obsessive compulsive disorder. The step of administering the SRI and the ondansetron is then repeated for more than seven days. In another method, an SRI, a neuroleptic, and ondansetron or a pharmaceutically acceptable salt thereof is administered to a patient suffering from obsessive compulsive disorder. The step of administering the SRI, the neuroleptic, and the ondansetron is then repeated for more than seven days. In another method, ondansetron or a pharmaceutically acceptable salt thereof is administered to a patient suffering from obsessive compulsive disorder for more than seven days. The ondansetron or pharmaceutically acceptable salt thereof may be administered as a pharmaceutically effective dose up to about 1.5 mg (free-base equivalent).
