Q: There’s a sticker on my prescription bottle that says not to eat grapefruit or drink grapefruit juice with it. I love grapefruit. Is it okay to have grapefruit juice for breakfast and take my pill in the evening instead?
No, because the “grapefruit effect” doesn’t wear off that fast. It can take up to 3 days for the effects of drinking a glass of grapefruit juice or eating a half a grapefruit on a particular medicine to wear off completely.
While many medicines should be taken at or near a meal to minimize the stomach upset they can cause, some are actually absorbed best on an empty stomach. The label on your prescription medicine doesn’t always have enough room to include this additional advice, so your pharmacist will attach a smaller label called an accessory label to the main one to call your attention to some additional, important information.
Some of these extra labels are an eye-catching color, and they’ll stand out because you notice them, while others end up on the back of the bottle. Many have such dastardly small letters that you can barely read them even when if you notice them.
The accessory label used for a potential problem with grapefruit says, “Avoid taking this medication with grapefruit or grapefruit juice”.
Okay, then, what could happen if you (gasp) forgot and had grapefruit juice for breakfast with that medicine? How long after taking your pill should you avoid having any grapefruit or grapefruit juice? And what’s the problem with grapefruit, anyway?
Grapefruit and grapefruit juice can change the way your body absorbs certain medicines. This “grapefruit effect” was discovered by accident. In 1991, researchers wanted to find out if drinking alcohol while taking a blood pressure medicine called felodipine would change the level of the medicine in the blood. To measure this, they created a study in which each participant had their blood pressure and the amount of medicine in their bloodstream measured several times, both with alcohol in their blood and without it.
The study was designed so that the participants could not tell which doses of the study medicine were given with alcohol, and which were not, so the researchers needed a way to disguise the taste of the alcohol so that the participants couldn’t tell if they were getting alcohol with their dose of medicine, or not.
The researchers chose grapefruit juice to disguise the taste of the alcohol and had each participant drink a small glass of grapefruit juice with each dose of the study medicine, half the doses with alcohol added and the other half without it.
Something curious happened. The study participants had much lower blood pressures than the researchers expected, along with blood levels of the medicine nearly 50% higher than they were used to seeing from the dose of medicine given. Drinking alcohol didn’t turn out to have much effect on the blood level of felodipine, but drinking grapefruit juice did!
What did the grapefruit juice do to the medicine to raise the blood level that high?
Grapefruit can change how medicine gets into your body. Although most pills dissolve in your stomach, they don’t actually get into your bloodstream from there. Vitamins, minerals, nutrients and medicines are transported into your bloodstream by your small intestine, which is lined with special cells and molecules working to make this happen. One special type of molecule is an enzyme called CYP3A4. Its job is to take apart and change the shape of certain medicines. Once this happens, the medicine stops working for you.
Grapefruit juice and fresh grapefruit contain natural compounds that inactivate the CYP3A4 enzymes lining your small intestine. This results in more medicine getting into your bloodstream because instead of being inactivated by your CYP3A4 enzymes, more medicine gets absorbed from your small intestine into your bloodstream, which increases its concentration in your blood and its effect on you.
However, this “grapefruit effect” is not the same with everyone, or with every medicine. Some medicines are not as affected by CYP3A4 enzyme as others are. In addition, there are genetic differences in how much CYP3A4 enzymes each of us have, and how active they are. Although this effect is mainly limited to grapefruit, two ounces of lime juice may cause a significant drug interaction in some people.
Drinking 7 ounces of grapefruit juice or eating one grapefruit can completely inactivate your intestinal CYP3A4 enzymes for up to 3 days. If you happen to be taking a drug affected by grapefruit and drink grapefruit juice or eat some grapefruit, you could get up to 50% more medicine than the dose prescribed for you by your doctor, even if you aren’t taking them at the same time.
With some medicines, this extra amount may not be noticeable, but older people may experience serious side effects or toxicity.
This “grapefruit effect” is limited to medicines taken by mouth, because although we also have a lot of CYP3A4 enzymes in our liver, only the ones in our small intestine are affected by eating grapefruit or drinking grapefruit juice.
Because one glass of grapefruit juice can affect certain medicines for up to 3 days, the only way to be completely safe when taking a medication with a grapefruit interaction is to avoid grapefruit juice and grapefruit entirely.
