When two or more medicines are taken at the same time, the effects of one of the medicines can be altered by the other. This is known as a drug-drug interaction. Warfarin can interact with many medicines.

If you want to check that your medicines are safe to take with warfarin, ask your doctor or local pharmacist, or read the patient information leaflet that comes with your medicine.

Lots of herbal medicines and supplements can interact with warfarin. Do not start taking any new herbal medicines or supplements without checking with your anticoagulant clinic, GP or pharmacist.

Aspirin and other painkillers

If you are on warfarin:

• do not take aspirin or drugs that contain aspirin because this could lead to bleeding 
• do not take ibuprofen
• you can take paracetamol but do not take more than the recommended dose

Interactions with food

Consult your GP before making any major changes to your diet while taking warfarin. They can also supply further information about foods to avoid or to limit.

Vitamin K

It is important to eat a consistent, healthy diet when taking warfarin.

Some foods, including liver and green leafy vegetables, such as spinach and broccoli, contain large amounts of vitamin K. When your first dose of warfarin is prescribed, it does not matter how much vitamin K you are eating as the dosage will be based around your current blood clotting levels.

However, if you make significant changes to your diet and begin to eat these foods in large amounts, or cut them out of your diet, this could interfere with how your warfarin works.

Cranberry and grapefruit juice

Cranberry juice can also increase the effects of warfarin, so you should avoid drinking it regularly if you are taking warfarin.

There is also some limited evidence to suggest that grapefruit juice can interfere with warfarin and cause a slight increase in your international normalisation ratio (INR, which is a measure of how long it takes your blood to clot). So again, it's best not to drink grapefruit juice regularly if you are taking warfarin. However, having the occasional small glass of cranberry or grapefruit juice is unlikely to cause any serious problems. 

Losing weight

Losing or putting on weight can also affect your INR. If you are taking warfarin you will need to be weighed regularly. See Considerations when taking warfarin for more information about INR.

Interactions with alcohol

It is dangerous to binge drink or get drunk while taking warfarin. Doing this may increase the effect of warfarin and so increase the risk of bleeding.

Do not drink more than three units of alcohol a day if you are a man or two units a day if you are a woman. It is not safe to save up units to have on one day.

One unit is roughly equivalent to half a pint of beer or lager or a single measure (25ml) of a spirit such as vodka. A small glass (125ml) of wine is around 1.5 units. Read more about units of alcohol.

Heavy drinkers or people with liver disease who are taking warfarin should not drink alcohol.