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).

CYP enzymes

A group of enzymes called cytochrome P (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. We know that certain ones affect how cancer drugs are broken down in the body. These include:

  • CYP3A
  • CYP4A
  • CYP2A
  • CYP5A

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 that affect the CYP enzymes are grapefruit and Seville oranges. This includes their juice and other products that are mostly made from these, for example marmalade.

The food and drink you may need to avoid depend on the drug you are having 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
  • mistletoe
  • milk thistle

For your safety

You must tell your doctor or pharmacist if you start a new medication or are thinking of taking some herbal supplements. They will be able to tell you if they might affect the CYP enzymes.

They will also be able to advise you about any foods, medications or any 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 take part in clinical trials if you are taking a medication that affects the CYP enzymes. Talk to your doctor or a member of the trial team if you are thinking of joining of 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

  • Effects of mistletoe products on pharmacokinetic drug turnover by inhibition and induction of cytochrome P450 activities

    Mi Schink and O Dehus

    BMC Complementary and Alternative Medicine , 2017. Volume 17, Issue 1: 521

  • Cytochrome P450: Polymorphisms and Roles in Cancer, Diabetes and Atherosclerosis

    I Elfaki and others

    Asian Pacific Journal of Cancer Prevention, 2018. Volume 19, Issue 8: 2057-2070

  • Food Bioactive Compounds and Their Interference in Drug Pharmacokinetic/Pharmacodynamic Profiles

    M Briguglio and others

    Pharmaceutics 2018. Volume 10: 277

  • 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: 
12 Aug 2019

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