Why has my doctor told me not to eat or drink some foods or take certain drugs with my cancer treatment?

This is because certain substances may 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).

We know from research that a group of body substances called cytochrome P (CYP) enzymes are a part of this process in the gut. There are several CYP enzymes, including a group called CYP3A. This includes a subgroup, the most important one for cancer drugs, called CYP3A4. Another enzyme in this group affects how some people absorb drugs and is called CYP3A5.

Some foods and other types of drugs can affect how CYP3A enzymes work. This can change the amount of cancer drug that you absorb. So it changes the dose of cancer drug that your doctor prescribed for you. This can affect how your cancer drug works, making it stronger or weaker.

One of the most commonly mentioned foods to avoid is grapefruit and its juice. There is a substance in grapefruit that blocks (inhibits) CYP3A. But this substance is found in other fruit including Seville oranges, pomegranate and star fruit.

The many different types of drugs that may affect how CYP3A enzymes work include

Certain antibiotics
Some chemotherapy drugs
Anti fungal drugs
HIV treatments
Drugs to stop fits (anti convulsants)
Drugs called calcium channel blockers
Antidepressants
Statins
Steroids

There are also many different herbal supplements that affect CYP3A enzymes. These include

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

Your doctor 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.

Your specialist will also know if a CYP3A enzyme affects the way your particular cancer drug is absorbed. If so, they may tell you to avoid, or at least limit your intake of certain foods, such as grapefruit. And you may need to stop taking some herbal supplements. Your doctor can advise you about this.

Some people will have another medical condition that means they can’t stop taking a drug that affects a CYP3A enzyme. In that 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.