COMT inhibitors like entacapone and tolcapone are used alongside L-Dopa to treat Parkinson's disease by reducing the breakdown of levodopa and increasing its effectiveness. Entacapone is generally well-tolerated but can cause side effects like diarrhea, vivid dreams, and drowsiness. Tolcapone is similarly effective at inhibiting COMT but carries a higher risk of liver toxicity and its use requires monitoring of liver function. Both drugs have contraindications when used together with other medications due to risk of interactions.