SRI's are considered first-line treatments for OCD, however many patients continue to have significant symptoms despite an adequate trial of an SRI. Neuroimaging studies have shown that the glutamate neurological system is involved in OCD. This study will test the safety and efficacy of topiramate, a drug, which targets glutamate, in the treatment of OCD, where the OCD has been un-responsive, or partially responsive to regular SRI treatment.

Inclusion Criteria:

• Outpatient with primary DSM- IV OCD
• Completion of a 14-week open label trial of one the following SRI's: fluoxetine 80 mg/day, paroxetine 60 mg/day, fluvoxamine 300 mg/day, clomipramine 250 mg/day, sertraline 200 mg/day, citalopram 60 mg/day, escitalopram 30 mg/day and demonstrating a non or partial responses to SRI treatment (CGI-I of 3 or 4, Y-BOCS reduction of < 35%)
• Stable (8 wks or longer) concurrent medications including benzodiazepines, sedative hypnotics, antipsychotics, and antidepressants.

Exclusion Criteria:

• Any other primary DSM-IV diagnosis; DSM-IV criteria for body dysmorphic disorder, bipolar affective disorder, schizophrenia, psychotic disorder, current alcohol/substance abuse.
• A previous adequate trial of topiramate
• Comorbid major depressive disorder diagnosis which predates OCD diagnosis
• Cognitive behavioural therapy or additional psychotherapy in past four months
• Allergy or hypersensitivity to topiramate
• BMI < 20
• History of kidney stones