PsilOCD: A Pharmacological-Challenge Feasibility Study

Key Inclusion Criteria: * Aged 20 to 65 years; * Any gender; * A primary diagnosis of OCD (based on the Mini-International Neuropsychiatric Interview (M.I.N.I.)); * Has met diagnostic criteria for OCD for at least 12 months; * Willing to comply with protocol and associated lifestyle restrictions; * Adequate understanding of the English language to give informed consent and participate in the study; * Participant can attend visits as an outpatient; * Comfortable using a computer, access to internet from home, and willing to participate in some of the study visits via video-link. Key Exclusion Criteria: * Current or past history of dependent (according to ICD10 criteria) substance use (not including nicotine and/or caffeine), Tourette's syndrome, autism spectrum disorder, epilepsy, organic mental disorder, or a personality disorder apart from obsessive-compulsive personality disorder; * Current or past history of psychosis or mania in themselves or a first-degree relative; * Unstable physical health; * Significantly abnormal clinical test result; * Heavy smoker, or unable to attend the dosing days (including the subsequent recovery part) without a smoking break; * Unwillingness to allow their GP or mental health practitioners to be informed of their participation (or, to allow study team access to Summary Care Record).