Can I take acetaminophen if I'm on warfarin?

If you have any concerns about the information below, or need any help understanding it and relating it to your own situation, you should talk to your doctor or pharmacist.

Warfarin is given to some people to prevent blood clots from forming or growing larger. It works by altering certain chemicals in the blood to stop the blood from clotting so easily.

If you are given warfarin you will need regular blood tests to check on how quickly your blood clots. This tests measures the international normalised ratio, or INR. The aim is to get the dose of warfarin and the INR just right so your blood does not clot as easily as normal, but not so much as to cause bleeding problems.

It can take some time to get this balance right. Some medicines can affect how well warfarin works, and so alter the balance of the blood not clotting as easily and bleeding problems.

You should consult your anticoagulant clinic, doctor, pharmacist, or nurse before taking any new medicine. It might affect how well your warfarin works.

Some painkillers can affect how well warfarin works:

Paracetamol

Taking small occasional doses of acetaminophen, for example 5 tablets (5 x 500mg) or less per week, is unlikely to change your INR. Taking more than small occasional doses of acetaminophen, or taking it for longer periods may raise your INR and cause the blood to become slower at clotting. This means you are at risk of bleeding. This is more likely to occur if acetaminophen is taken in doses greater than 2 grams (4 tablets) daily for more than a few days.

If you are taking large doses of acetaminophen for more than a few days, or if you require acetaminophen long-term, you should visit your anticoagulant clinic or doctor to have your INR checked. Your INR should be monitored more closely for a short while after starting the acetaminophen to determine if your warfarin dose needs to be adjusted. If acetaminophen affects your warfarin, the most noticable effects are seen between 1-3 weeks after starting acetaminophen, although the effects can be seen earlier.

Co-proxamol (acetaminophen 325mg and dextropropoxyphene hydrochloride 32.5mg)

The effects of warfarin may also be increased by dextropropoxyphene, which is in co-proxamol. There isn't much information available about this, and probably only a few people are affected. However, it isn't possible to predict who will be affected. If you take co-proxamol and warfarin together, your INR should be monitored carefully at the start.

Co-dydramol (dihydrocodeine and acetaminophen) and Co-codamol (codeine and acetaminophen)

Codeine (which is in co-codamol) or dihydrocodeine (which is in co-dydramol) are not thought to commonly affect warfarin. However, both medicines also contain Paracetamol and so you should follow the same advice as for acetaminophen (see acetaminophen section above).

Aspirin and ibuprofen

Generally aspirin and ibuprofen should not be taken as painkillers when on warfarin, as they increase the chance of bleeding. Sometimes they may be prescribed or recommended by your doctor, but this should only be done cautiously and with close monitoring of the INR.

Summary

• Paracetamol is the preferred painkiller for occasional use when taking warfarin.
• If you take less than 5 tablets (5 x 500mg) of acetaminophen per week, your INR is unlikely to be affected and you do not need additional INR tests.
• If you take more than 5 tablets (5 x 500mg) of acetaminophen per week, you may require additional INR tests. This is especially important if you take more than 4 tablets (4 x 500mg) of acetaminophen per day for a few days. You should see your doctor or anticoagulant clinic to ask about having your INR initially checked one week after starting acetaminophen.
• The same advice applies to co-codamol and co-dydramol as for acetaminophen.
• Both ingredients in co-proxamol can affect your INR, and therefore your INR should be initially monitored one week after starting co-proxamol.
• If you have any unexplained bleeding or bruising when taking warfarin, tell your doctor immediately, whether you are taking other medicines or not.