COMT inhibitors are used to treat Parkinson's disease. They work by blocking catechol-O-methyltransferase (COMT), an enzyme (protein) that breaks down dopamine in the body. COMT inhibitors help levodopa-based medications last longer in the body. So you'll take them along with carbidopa / levodopa medications for Parkinson's disease. Examples of COMT inhibitors include entacapone (Comtan), opicapone (Ongentys), and tolcapone (Tasmar).