Different medications have different directions when it comes to taking them. Take with food, take on an empty stomach, take before sleeping, blah, blah, blah. Most people like to take their pills with a beverage of some sort to wash it down, but there’s one seemingly innocuous drink you should be especially cautious about mixing with your medicine.

Grapefruit juice is often singled out on medicine warning labels, but what gives? What makes grapefruit juice different from any other juice you may use to wash down a pill? The answer lies with a pesky organic compound found in grapefruit juice called furanocoumarin. This compound messes with an enzyme in your intestine called CYP3A4, which can change the rate at which your body absorbs certain medicines, including ones meant for high cholesterol, high blood pressure, and anxiety.

Normally, the enzyme keeps a large part of many medications from entering your body and doing their jobs, something that is take n into account when companies produce drugs and decide on appropriate dosages. If you drink a glass of grapefruit juice with the medicine, however, it takes out almost half of the enzyme—meaning a lot more medicine is being absorbed into your body than it should be. This can result in dangerous amounts of medication entering your body, putting you at risk for adverse effects.

Check out the video to see how the process works!