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Food and drink to avoid during cancer treatment

Certain substances in foods and drugs can interfere with how the cancer drug you are having works.

To be able to work, all drugs, including cancer drugs, must be broken down and absorbed into the body. When you take medications by mouth as tablets, capsules or liquids, this process happens in the gut (digestive system).

CYP3A enzymes

We know from research that a group of body substances called cytochrome P (CYP) enzymes are a part of the absorption process in the gut.

There are several CYP enzymes, including a group called CYP3A. This group includes enzymes called CYP3A4 and CYP3A5. They are important in how some people absorb drugs. 

Some foods and drugs affect CYP3A

They can affect the amount of cancer drug that you absorb. This means that your doctor needs to change the dose they prescribe for you. This can affect how your cancer drug works, making it stronger or weaker.

Foods to avoid

Grapefruit is one of the most commonly mentioned foods to avoid - there is a substance that blocks (inhibits) CYP3A. But this substance is also found in other fruit.

  • grapefruit and its juice
  • seville oranges
  • pomegranate
  • star fruit


There many different types of drugs that may affect how CYP3A enzymes work including:

  • certain antibiotics
  • some chemotherapy drugs
  • anti fungal drugs
  • hiv treatments
  • drugs to stop fits (anti convulsants)
  • drugs called calcium channel blockers
  • anti depressants
  • statins
  • steroids

Herbal supplements

There are also many different herbal supplements that affect CYP3A enzymes, including:

  • St John’s Wort
  • Black cohosh
  • Ginseng
  • Ginko biloba
  • Goldenseal

Your doctor will monitor your drug absorption

Your doctor or pharmacist will be able to tell you if you are taking any drugs or supplements that may affect the CYP3A enzymes and so could affect your cancer treatment. They will advise whether you need to stop certain foods or herbal supplements

You may have another medical condition that means you can’t stop taking a drug that affects a CYP3A enzyme. If this is the case, your doctor might need to change the dose of your cancer drug.

This also means that you may not be able to take part in clinical trials looking at cancer drugs that could be affected by CYP3A enzymes.

Last reviewed: 
19 Jan 2015
  • Drug interactions in cancer therapy
    Charity D. Scripture and William D. Figg
    Nature Reviews Cancer 6, 546–558 (2006)

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