Carbidopa/oxitriptan, or carbidopa/5-hydroxytryptophan (carbidopa/5-HTP), is a combination of 5-hydroxytryptophan, a serotonin precursor, and carbidopa, a peripherally selective aromatic L-amino acid decarboxylase (AAAD) inhibitor, which is under development as an add-on therapy for the treatment of obsessive–compulsive disorder (OCD) in people taking a selective serotonin reuptake inhibitor (SSRI). It is or was also under development as an antidepressant for the treatment of depressive disorders in people taking an SSRI or serotonin–norepinephrine reuptake inhibitor (SNRI) with an inadequate treatment response, but no recent development has been reported for this indication.