Food and drink to avoid during cancer treatment

Taking medicines at home is a common part of cancer treatment. It is important to take the medicine safely and as instructed.

Everything you take by mouth, including food, drink and medicine, needs to be absorbed by the body. This process happens in the gut (digestive system).

CYP450 enzymes

A group of enzymes called cytochrome P450 (CYP) are an important part of the process of how drugs are broken down in the body after they’ve been absorbed into the blood. Most of these enzymes are made by the liver.

There are many CYP enzymes. The following enzymes are responsible for breaking down 90 out of every 100 (90%) drugs in the body:

  • CYP3A4
  • CYP2D6
  • CYP2C9
  • CYP2C19
  • CYP1A2

The amount of these enzymes in the body can affect how well the cancer drug is broken down. This affects how well the drug works and the possible side effects.

If too much of the cancer drug is broken down it might not work as well, and you might need a bigger dose which can possibly cause more side effects. If too little of the drug is broken down, you could have bad side effects from your cancer drug and you might need a smaller dose.

Foods and other drugs, including herbal supplements, can affect the level of CYP enzymes in the body. So while you are taking certain drugs, your healthcare team may tell you to avoid some foods or herbal supplements. You may also read this in the drug information sheet enclosed with your medicine. Check with your doctor or pharmacist if you’re not sure.

Foods that affect CYP enzymes

The best known foods affecting CYP enzymes are grapefruit and Seville oranges. This includes their juice and other products mostly made from these, for example, marmalade.

The food and drink you may need to avoid depend on the drug you have and which CYP enzymes are involved. Your doctor or pharmacist will tell you if you need to avoid particular foods or drink while on treatment.

Drugs that affect CYP enzymes

There are many different types of drugs that can affect the CYP enzymes. These include:

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

Herbal supplements that affect CYP enzymes

There are many herbal supplements that affect the CYP enzymes. Some of the known ones include:

  • St. John’s Wort
  • black cohosh
  • ginseng
  • gingko biloba
  • goldenseal
  • milk thistle

For your safety

You must tell your doctor or pharmacist if you start a new medication or are considering taking herbal supplements. They can tell you if they might affect the CYP enzymes.

They will also be able to advise you about any foods, medications or supplements that might affect these enzymes.

You might have another medical condition for which you are taking medication that might affect the CYP enzymes. If this is the case, your doctor might need to change the dose of your cancer drug.

Taking part in clinical trials

You might not be able to participate in clinical trials if you take a medication that affects the CYP enzymes. Talk to your doctor or a trial team member if you are considering joining a clinical trial. If possible, you might be able to join the trial if you can take another medication that doesn’t affect the CYP enzymes.

  • The mechanisms of pharmacokinetic food-drug interactions – A perspective from the UNGAP group

    M Koziolek and others

    European Journao of Pharmaceutical Sciences, 2019. Issue 134: 31-59

  • Biochemistry, Cytochrome P450

    B Gilani and M Cassagnol

    In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 January

  • Food-drug interactions precipitated by fruit juices other than grapefruit juice: An update review

    M Chen and others

    Journal of Food and Drug Analysis. 2018 April. Volume 26 Issue 2S, Pages: S61 to S71

  • The information on this page is based on literature searches and specialist checking. We used many references and there are too many to list here. If you need additional references for this information please contact with details of the particular issue you are interested in.

Last reviewed: 
30 Jun 2023
Next review due: 
30 Jun 2026

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