Antidepressants or antipsychotic medications may be used for more severe cases if intrusive thoughts do not respond to cognitive behavioral or exposure therapy alone. Whether the cause of intrusive thoughts is OCD, depression, or post-traumatic stress disorder, the selective serotonin reuptake inhibitor (SSRI) drugs (a class of antidepressants) are the most commonly prescribed. Intrusive thoughts may occur in persons with Tourette syndrome (TS) who also have OCD; the obsessions in TS-related OCD are thought to respond to SSRI drugs as well.