A refreshing slice of grapefruit or a tall glass of its juice can have many benefits; it’s packed with vitamins C and A, as well as helpful minerals like potassium. But before you go and pick up 10 cartons of the stuff, there’s an important downside to be aware of – grapefruit can also mess with a whole mass of different medications.

There are too many to name them all here – but to give you an idea of the range, it can affect medications used to treat high cholesterol, anxiety and depression, heart problems, and organ transplant rejection.

How are these drugs affected by consuming grapefruit? In one of two ways.

Too much of the drug

For most of the drugs affected by grapefruit, it can result in abnormally high levels of the medication in the bloodstream. This is because grapefruit juice interacts with an enzyme in the intestines called cytochrome P450 3A4 (CYP3A4), which is responsible for breaking down many different types of drugs.

Compounds within grapefruit known as furanocoumarins are also broken down by CYP3A4 – but the resulting products then go on to bind to, and irreversibly block the action of, the enzyme. As a result, more of the medication the enzyme usually acts upon ends up in the bloodstream, and normal function is only restored once the body makes more CYP3A4.

People have different levels of CYP3A4 in their intestines, so the impact of consuming grapefruit can vary between individuals. In some cases, the level of a drug that ends up in the bloodstream can have serious side effects.

For example, in one case study, a patient who had received a liver transplant and was taking the drug tacrolimus to reduce the risk of rejection wound up back in hospital with acute kidney dysfunction. Why? 

It turns out they’d eaten more than 1.5 kilograms (3.3 pounds) of marmalade over the course of a week (this was definitely a study of a human and not a bear from Peru) and it just so happened that marmalade was more than half made up of grapefruit. 

As a result, the patient had a 326 to 592 percent greater concentration of the drug in their bloodstream than there should’ve been, which led to the effects seen in the kidneys.

Too little of the drug

It’s also been found that grapefruit can end up leading to too little of a drug in the body, this time by targeting a different group of proteins: transporter proteins that move medications into the bloodstream from the intestines, blocking them from being able to do so.

One of the most common examples of a drug affected by this is fexofenadine, a type of antihistamine that’s used to treat hay fever and hives. “Grapefruit juice can cause less fexofenadine to enter the blood,” said Dr Shiew Mei Huang, Deputy Director of the FDA’s Office of Clinical Pharmacology, in an FDA consumer update.

The consequence of this is that drugs like fexofenadine can end up being less effective than intended – not exactly what you want when the pollen bomb cometh.

The bottom line

If you aren’t sure about whether grapefruit will affect your medication, check the information sheet that comes in the packet, as this usually lists things to avoid. If it doesn’t say – or you’ve thrown the sheet away (we’ve all done it) – a clinician or pharmacist will be able to tell you more.

While there, it can be a good idea to check about other food and drink that might cause problems; Seville oranges, pomelos, and tangelos can have similar effects to grapefruit. 

In cases where grapefruit and medications can’t mix, keep an eye out for the fruit and its juice on labels – as anyone with an allergy or intolerance will tell you, these things can pop up in the most surprising of places.

It might be tempting to have some grapefruit anyway if it's been a while since your medication was taken, but its effects can last for a surprisingly long time. In the case of felodipine, a drug used to treat high blood pressure, grapefruit can still affect its action even if consumed a whole day before.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.  

 The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.