Tolcapone inhibits the catechol-O-methyltransferase (COMT) enzyme, which degrades dopamine and levadopa, thereby prolonging the therapeutic effects of levodopa. It, alongside inhibitors of peripheral dopa decarboxylase, have been used to complement levodopa. However, due to its possible side effects such as liver failure, it is limited in its availability. A similar drug, entacapone, has not been shown to cause significant alterations of liver function and maintains adequate inhibition of COMT over time. Entacapone is available for treatment alone (COMTan) or combined with carbidopa and levodopa (Stalevo).