Methods for treating obsessions and/or compulsions are described. In one method, ondansetron or a pharmaceutically acceptable salt thereof is administered to a patient suffering from obsessions and/or compulsions 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). In another method, a serotonin reuptake inhibitor (SRI) and ondansetron or a pharmaceutically acceptable salt thereof is administered to a patient suffering from obsessions and/or compulsions. The method may optionally include the step of administering a serotonin reuptake inhibitor to the patient. Additionally or alternatively, the method may also include the step of administering a neuroleptic to the patient.
