The enzyme catechol-O-methyl transferase (COMT) is responsible for the breakdown of levodopa. Therefore, co-administration of levodopa with a COMT inhibitor prolongs the therapeutic actions of levodopa and reduces “off” time where the motor symptoms of Parkinson’s disease cannot be adequately controlled. Two reversible COMT inhibitors, entacapone and tolcapone, have been approved for clinical use in patients, and a third, nebicapone, is in development. More information about COMT inhibitors is available on the EPDA website.