Grapefruit and grapefruit juice are often praised as a healthy, tasty addition to the diet. They’re rich in vitamin C, potassium, and fiber, among other nutrients (1). But what many people don’t realize is that grapefruit juice can also interfere with a surprising number of medications, including common treatments for high blood pressure, high cholesterol, and certain psychiatric conditions, sometimes with serious or even dangerous consequences (1).
Origins
of the Grapefruit Effect
 |
| Figure 1. Dr. David Bailey, who first accidentally discovered the interaction between grapefruit and felodipine. Since his initial finding, Dr. Bailey and his colleagues have published numerous studies on grapefruit's effects on drug metabolism (5). |
Inside the Body: How the Interaction Works
When you swallow a pill, your body has several
ways of making sure the drug doesn’t hang around forever. The most important of these is metabolism, especially by enzymes in
the cytochrome P450 family.
One member, CYP3A4,
does a lot of heavy
lifting; it helps
break down roughly half of
all prescription drugs (4). You’ll find this enzyme both in the cells lining
your small intestine and colon (enterocytes) and in the main cells of the liver
(hepatocytes). Because of that, many oral drugs go through a sort of “double
pass” of metabolism, first in the gut wall and then again in the liver, before
they ever reach your bloodstream. This “first-pass” effect can dramatically
reduce how much of a drug actually gets into circulation. For instance, only
about 15% of a swallowed dose of felodipine survives this process unchanged,
which means it has naturally low oral bioavailability (4).
 |
| Figure 2. First-pass metabolism of felodipine. After absorption in the small intestine, the drug is broken down first by the intestinal cells (enterocytes) and then by liver cells (hepatocytes). Although 100% of felodipine is absorbed from the gut, only about 15% reaches the bloodstream unchanged (CYP3A4 = cytochrome P450 enzyme 3A4)(4). |
That’s where grapefruit becomes important. Grapefruit contains furanocoumarins, compounds that don’t just slow down CYP3A4, they permanently inactivate the enzyme in your intestinal lining by binding to its active site (a process called mechanism-based inhibition) (4). The enzyme then has to be newly synthesized before activity returns to normal. This results in much less drug breakdown in the gut, and much higher peak blood levels and greater overall exposure for affected medications (4). Notably, the elimination half-life and the handling of intravenous drugs remain unchanged, because the liver’s metabolism is less affected (4).
This interaction isn’t limited to one form of grapefruit. Fresh juice, frozen concentrate, or even a whole fruit can all knock down CYP3A4 activity, and just one whole grapefruit or a glass (about 200 mL) of juice may be enough to produce a clinically significant effect. Other citrus fruits like Seville oranges (used in marmalade), limes, and pomelos contain similar compounds and can cause the same problem. In contrast, sweet oranges such as navel or Valencia lack furanocoumarins and don’t interfere with CYP3A4 (4).
Conclusion
Grapefruit is packed with nutrients and tastes great, but its interaction with common medications can end up causing unwanted side effects. By knocking out a key drug-metabolizing enzyme in the gut, it can quietly push the levels of many common medications far higher than intended, sometimes to dangerous levels. And because even a single glass of juice can have an effect that lasts a day or more, there is a real risk to patients taking drugs broken down by CYP3A4 (4).
So the next
time you’re prescribed a new medication, ask your doctor or pharmacist whether grapefruit (or related citrus)
is safe to have with it. A quick conversation can prevent deadly
consequences. And if you’re craving citrus with breakfast? Pick up a sweet
orange instead, your taste buds and your medications will thank you.
By Madeline Sutherland, a Master's of Medical Sciences Student at the University of Kentucky
References
1. Publishing, H. H. (2021, March 30). Grapefruit and medication: A cautionary note. Harvard Health.https://www.health.harvard.edu/staying-healthy/grapefruit-and-medication-a-cautionary-n ote
2. Bailey, D. G., Malcolm, J., Arnold, O., & David Spence, J. (1998). Grapefruit juice–drug interactions. British Journal of Clinical Pharmacology, 46(2), 101–110. https://doi.org/10.1046/j.1365-2125.1998.00764.x
3. Bailey, D. G., Spence, J. D., Edgar, B., Bayliff, C. D., & Arnold, J. M. (1989). Ethanol enhances the hemodynamic effects of felodipine. Clinical and Investigative Medicine, 12(6), 357–362. https://pubmed.ncbi.nlm.nih.gov/2612087/
4. Bailey, D. (2013). Appendix 1: Grapefruit Interacting Drugs and Associated Oral Bioavailability, Adverse Event(s), Risk Ranking and Potential Alternative Medications Interacting Drugs Innate Oral Bioavailability* Dose-Related Drug Adverse Event(s) Predicted Interaction Risk Rank ** Potential Alternative Medication(s)*** Anti-Cancer. Canadian Medical Association Journal. https://doi.org/10.1503/cmaj.120951
5. Stewart, M. (2022,
September 23). David Bailey, Olympian and
pharmacologist who discovered the grapefruit effect, dead at age 77. The
Globe and Mail. https://www.theglobeandmail.com/canada/article-david-bailey-olympian-and-pharmacolo gist-who-discovered-the-grapefruit/
