“May cause drowsiness.” “Do not drink alcoholic beverages while taking this medication.” Most warnings on prescription bottles are pretty easy to understand. But “Do not eat grapefruit or drink grapefruit juice at any time while taking this medication”? What the heck is that about?

Enzymes, friends. It’s about enzymes. Or rather, one enzyme in particular: CYP3A4, which helps your body break down, absorb, and use many different drugs.

The grapefruit (Citrus x paradisi) and its juice contain chemicals that interfere with CYP3A4 and keep it from doing its job properly. Depending on the medication, it can either cause your body to take in and retain too much of the drug, leading to a potential overdose, or not enough, rendering your medication useless.

It’s not just grapefruit, either.

“There are many things that can effect drug efficacy,” pulmonary and critical care specialist Dr. Rob Menzies tells mental_floss.

Seville oranges (the kind often used in marmalade) and tangelos are CYP3A4 blockers, too. Dairy products and green, leafy vegetables like spinach and kale can mess with other metabolic pathways.

And then there’s all your other prescriptions.

“The biggest issue is from polypharmacy (taking multiple medications at the same time),” Menzies says, “as many drugs will have effects on other drugs.”

Check your medication bottles and take those warnings seriously; they’re not just there because someone in the pharmacy got sticker-happy.

If you have questions about how your prescriptions will interact with a specific food or other drugs or supplements, talk to your doctor or pharmacist.

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