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COMT Inhibitors

This class of PD medications includes entacapone (Comtan®) and tolcapone (Tasmar®). They have no direct effect on PD symptoms, but are used to prolong the effect of levodopa by blocking its metabolism. COMT inhibitors are used primarily to help with “wearing-off,” in which the effect of levodopa becomes short-lived. People taking Tasmar must have regular liver function blood tests. Entacapone is not only a COMT inhibitor, but is also a main ingredient in Stalevo, which is a combination of carbidopa, levodopa and entacopone.

What are the facts?

  • Only effective when used in combination with levodopa.
  • When taking levodopa, an enzyme in the body call catechol-O-methyl transerase (COMT) converts a portion of the levodopa into a form that is useless.
  • Block the COMT enzyme from converting levodopa into a useless form, thus making more levodopa in the brain available and helping to reduce PD symptoms.
  • Due to earlier instances of liver function abnormalities during clinical trials of tolcapone, the FDA mandates that blood tests of liver function be conducted at the beginning of the treatment and, every six months after beginning treatment.

What are the side effects?

  • May exaggerate some levodopa-related side effects especially dyskinesia
  • Confusion
  • Hallucinations
  • Discoloration of urine (reddish brown or rust-colored)
  • Diarrhea

Caution: PD medications may have interactions with certain foods, other medications, vitamins, herbal supplements, over the counter cold pills and other remedies. Anyone taking a PD medication should talk to their doctor and pharmacist about potential drug interactions.


Available Doses

Initial Dosing

Side Effects*





200 mg

200 mg with levodopa; max 8 per day

Abdominal pain, back pain, constipation, nausea, diarrhea, blood in urine

Secondary medication; delays wearing off by prolonging effectiveness of levodopa

MAO inhibitors



100 mg

200 mg

100 mg 3X/day

Abdominal pain, back pain, constipation, nausea, diarrhea, blood in urine, liver failure

Tertiary medication for motor fluctuations; limited in use to those who have exhausted other treatment options

MAO inhibitors

* Please note that the side effects listed in the tables that accompany each class of medication are the most commonly experienced. Not all individuals will experience such side effects. For many people who do experience side effects, they can often be effectively limited or eliminated with careful adjustments to dosage or the timing of the individual doses.

Speak to the treating physician immediately if any side effects are experienced. For a complete description of each drug and its possible side effects, please request a “package insert” from your pharmacist for each drug used. It is recommended that all prescriptions be filled at the same pharmacy to avoid interactions between medications. Interactions can be dangerous and even life-threatening, so make sure the pharmacist knows of all medications and supplements being taken, including over-the-counter medications and supplements.

Page reviewed by Dr. Chauncey Spears, Movement Disorders Fellow at the University of Florida, a Parkinson’s Foundation Center of Excellence.


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