Grapefruit Juice and Cancer Medicines: Is It Safe?

Most food and drug warnings are vague. This one is not, and the Food and Drug Administration explains why in plain terms.

Many medicines are broken down by an enzyme called CYP3A4, which sits in the small intestine. Grapefruit juice blocks that enzyme. So instead of part of the dose being broken down before it reaches the bloodstream, more of the drug gets through and stays in the body longer.

FDA's summary of the result: too much drug in your body. And as the agency's expert puts it, when there is too much drug in the blood, you may have more side effects.

This is not folklore. FDA has required some prescription and over-the-counter medicines taken by mouth to carry warnings against grapefruit juice and grapefruit.

FDA's consumer page lists examples from statins, blood pressure drugs, transplant drugs, and others. It does not list cancer drugs. That can leave the impression that this is somebody else's problem.

It is not. Take venetoclax, an oral drug used in some blood cancers. Its FDA-approved label tells patients directly: you should not drink grapefruit juice, eat grapefruit, Seville oranges, or starfruit during treatment, because these products may increase the amount of the drug in your blood.

The prescribing information gives the reason in the same language FDA uses — these fruits contain inhibitors of CYP3A.

That is what a real warning looks like. If your oral cancer drug has one, it will be in the leaflet in the box.

FDA names three that might behave like grapefruit:

• Seville oranges, the bitter ones often used for marmalade.
• Pomelos.
• Tangelos, a cross between tangerines and grapefruit.

Its instruction is straightforward. Do not eat those fruits if your medicine interacts with grapefruit.

Some drug labels go further than the consumer page. Venetoclax adds starfruit. So the fruit list is worth checking against your own label rather than a general article.

FDA also flags something easy to miss at the store: if you must avoid grapefruit juice, check the labels of juices and fruit-flavored drinks to see whether grapefruit juice is in the blend.

The interaction does not always run one way, which is worth knowing because it breaks the simple mental model.

FDA describes the opposite effect with fexofenadine, an allergy medicine. Grapefruit juice can cause less of it to enter the blood, so it does not work as well. That happens through drug transporters rather than through the enzyme. Its label says not to take it with fruit juices, and orange and apple juice can do the same thing.

So "grapefruit raises drug levels" is a useful rule of thumb, not a law.

People often ask whether a small glass, or a glass at a different time of day, would be fine.

FDA does not answer that with a number, and it explains why. The severity of the interaction can differ depending on the person, the drug, and how much grapefruit juice you drink. The amount of CYP3A4 in the intestine varies from person to person — some people have a lot of it and others have very little, so the same drug and the same juice can affect two people differently.

FDA's own list of things to find out from your provider or pharmacist includes exactly this: how much, if any, grapefruit juice you can have.

Which means the honest answer to "is a little okay?" is that only your pharmacist, holding your label, can say.

• Do any of my medicines interact with grapefruit?
• Does the restriction include Seville oranges, pomelos, tangelos, or starfruit?
• How much, if any, can I have?
• Does this apply to any of my non-cancer medicines too?

Read the leaflet that comes with an oral cancer drug. FDA points out that the medication guide or patient information sheet is where this is spelled out, and that over-the-counter Drug Facts labels say whether fruit juices are a problem.

This is general information. Your care team's specific instructions for your treatment always take priority.

• FDA — Grapefruit Juice and Some Drugs Don't Mix
• DailyMed — VENCLEXTA (venetoclax) prescribing information