Q: There’s a sticker on my prescription bottle that says not to eat grapefruit or drink grapefruit juice. How long do I have to avoid grapefruit when taking my medicine?
Some medications are best taken in a particular way: either with food or taken before meals. Other medicines need to be refrigerated or shaken well before they are poured out.
The main label on a prescription bottle doesn’t always have space to include this additional and vital advice. Your pharmacist will add a small sticker called an accessory label to your medicine bottle to draw attention to this critical information.
Some medicines come with an accessory label that talks about grapefruit: “Avoid taking this medication with grapefruit or grapefruit juice.” But it doesn’t say precisely what would happen if you did. What if you forget and have grapefruit juice for breakfast when you take that pill? And how long should you avoid grapefruit juice after taking it?
Why should you avoid mixing grapefruit juice with some medicines?
It’s because grapefruit juice can change the way your body absorbs certain types of medicines. Most pills dissolve while in your stomach, but that’s not where most of the action comes to getting the dose into your body. The lining of your small intestine is full of blood vessels and essential molecules that move vitamins, minerals, nutrients, and medicines into your bloodstream and throughout your body.
One type of unique molecule found in the lining of your small intestine are proteins that can break apart medicines and change their shape. This process changes how some medications work.
One protein, called CYP3A4, can take drugs apart, inactivating them before your body can absorb and utilize them. Several dozen medicines are affected by this enzyme, some more than others.
Grapefruit and fresh grapefruit juice contain natural compounds that inactivate the CYP3A4 enzymes lining your small intestine. Without CYP3A4, less medicine is inactivated, and more is absorbed into your body.
When you eat or drink grapefruit, it can allow up to 50% more medicine into your body, like taking an extra half tablet of it every day. This additional amount may not be noticeable with some drugs. With heart medicines, 50% more medicine every day can cause serious side effects or toxicity.
This “grapefruit effect” was discovered by accident. In 1991, researchers wanted to determine whether drinking alcohol while taking a particular blood pressure medicine would affect someone’s mental alertness or change the concentration of drug in the blood.
To measure this, they designed a study where each participant took the blood pressure medicine felodipine several times, sometimes with alcohol and sometimes without.
It was critically important to keep the participants from realizing whether they were getting alcohol or not. It would affect the results.
The researchers asked, “How can we mask the alcohol taste so that the participants can’t tell when we are giving it to them?”
Their solution was grapefruit juice. Each participant received a glass of grapefruit juice with their medicine, one half spiked with alcohol, and the rest drinking it plain.
Something curious happened. The study participants all had lower blood pressures than the researchers expected to see. Looking further, their blood levels of the study medicine were 50% higher than usual. Drinking alcohol while taking the study medicine didn’t affect the drug’s concentration in the blood, but drinking grapefruit juice sure did!
Further testing has shown that drinking only 7 ounces (less than a typical glass) of grapefruit juice completely inactivates the CYP3A4 enzymes in your small intestine for up to 3 days. If you take felodipine for your high blood pressure, you’ll get 50% more medicine than the dose prescribed for you until your enzymes recover.
Is this “grapefruit effect” affect everyone?
No. Some people have fewer CYP3A4 enzymes in their small intestine, so inactivating them won’t increase the level of medicines as those with more. Some medicines are not as affected by CYP3A4 as others.
Do other citrus fruit have this effect?
Luckily, no. Oranges, lemons, and limes do NOT seem to have this effect on medications and are perfectly safe to have instead of grapefruit.
If you miss your grapefruit juice in the morning, can you avoid the interaction by taking your medicine later in the day?
No, because the effects of a glass of grapefruit juice on certain medicines can last up to 3 days. The only way to be completely safe when taking a pill with a grapefruit interaction is to avoid grapefruit juice and grapefruit entirely.
