COMT inhibitors

COMT inhibitors can block an enzyme that breaks down levodopa - a key drug used to treat Parkinson's - helping it to work more effectively.

How do COMT inhibitors work?

Dopamine is a chemical messenger made in the brain. The symptoms of Parkinson’s appear when your dopamine levels become too low. This is because many of the cells in your brain that produce dopamine have died or are dying.

Unfortunately, taking dopamine as a drug treatment for Parkinson’s doesn’t help, because it can’t cross into your brain where it’s needed. However, you can boost the supply in your brain by taking levodopa, a drug that acts as a chemical building block to help your body produce more dopamine.

COMT inhibitors can block an enzyme that breaks down levodopa.

When are COMT inhibitors used?

These drugs are used alongside levodopa, to help it work more smoothly. They can be tried if your dose of levodopa is not working for long enough.

COMT inhibitors do not help to manage the symptoms of Parkinson’s on their own – they have to be used with levodopa.

COMT inhibitors come as tablets. If you take entacapone, you should take it at exactly the same time as your levodopa medication to get the best results.

There is also a combined tablet that includes levodopa, carbidopa (one of the ‘helper’ drugs that are always given alongside levodopa) and the COMT inhibitor entacapone.

What types of COMT inhibitor are there?


  • Comtess (tablets)

Co-careldopa plus entacapone

  • Stalevo (tablets)
  • Sastravi (tablets)


  • Tasmar (tablets)


  • Ongentys (tablets)

Benefits of COMT inhibitors

Helping levodopa work more smoothly

COMT inhibitors can help when levodopa is not working for long enough and starts to wear off between doses.

They can help to reduce your ‘off’ time, when you have more trouble with your Parkinson’s symptoms, and increase the amount of ‘on’ time, when your symptoms are better controlled.

Taking less levodopa

In many cases, levodopa can be taken less often and at a lower dose.

Improving Parkinson’s symptoms

Sometimes, adding a COMT inhibitor to your medication regime may help your Parkinson’s symptoms.

Risks and side effects of COMT inhibitors

Liver damage

With tolcapone, there is a risk of liver damage that can be fatal. It is rare, but for this reason, it is only used if you can’t take entacapone, where the risk of liver damage has not been seen.

If you take tolcapone, you will need regular blood tests to check the health of your liver.

Worse levodopa side effects

Because COMT inhibitors strengthen levodopa, the side effects of levodopa, including involuntary movements and sickness, can get worse too.

If this happens, your specialist or Parkinson’s nurse can reduce your dose of levodopa.

other side effects

The patient information leaflet that comes with your medication will tell you the full range of side effects that you may experience.

Some of the possible side effects include:

  • sleeping problems
  • loss of appetite
  • diarrhoea
  • dizziness
  • fainting
  • falls
  • hallucinations
  • headaches
  • confusion
  • dry mouth
  • chest pain
  • sleepiness

These drugs can also discolour your urine.

Find out more about the side effects of Parkinson's drugs.

Last updated August 2015. We review all our information within 3 years. If you'd like to find out more about how we put our information together, including references and the sources of evidence we use, please contact us at [email protected].