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Effects of Caffeine and Grapefruit Juice

Written by Phil

A coffee enthusiast and traveler, Phil primarily spends his days thinking and writing about coffee. He also writes for where he explores ideas of culture, psychology, and travel, and occasionally dabbles in horror.

After reading one of Tim Ferriss’ books, I came across the idea that grapefruit juice has some special properties that can double the time that caffeine stays in your system. It seems plausible: grapefruit juice is known to interact with several common medications. So what’s the truth about mixing caffeine and grapefruit? Should we consider combining the effects of caffeine and grapefruit while burning the midnight oil?

Research shows no significant effect found from mixing caffeine and grapefruit juice (or naringin). While it significantly affects many common medications, caffeine doesn’t appear to be one of them.

Well, so much for getting more bang out of your morning java.

What’s so special about Grapefruits and their Juice?

You’ve probably heard the above – grapefruit can cause complications with various drugs, one of the more impactful might be the birth control pill.

The effect is often believed to be a result of a compound called naringin, which also gives grapefruit it’s distinctive bitterness. Some believe that it causes tongue receptors to become more active, so they will eat a bite of grapefruit before a meal. This, I can’t confirm.

Naringin can interfere with the ability of enzymes to do their job, slowing the ability to break down various compounds. As a result, drugs end up in higher concentrations in the blood, making the effect stronger and longer-lasting. This seems to be the mechanism that is believed to affect caffeine.

Metaphor: imagine there’s a doctor’s office with a few people waiting. More people enter, and some are taken out. Over time, the room becomes less full as each patient gets processed faster than new ones appear, allowing the office to remain comfortable.

Suddenly, many pushy, impatient people come in. They shout at reception, blocking walkways, and consume the doctors’ time. The flow of patients slows to a standstill, causing the room to become crowded.

This is bad for the operation of the office, but so long as the people in the waiting room are calm and stable, no real issue. However, if the room is full of caustic, angry people, you’re in for trouble.

Caffeine would be one of the ones that is relatively well-adjusted, but you’d need to keep an eye on your other medications and check with your doctor to make sure they’re not tough customers.

I’ll point out this analogy is the belief of why grapefruit juice affects drugs, including caffeine. However, that doesn’t seem to be the whole story.

Whole Food Advantage

While many people want to isolate a single compound and take a supplement, it seems that taking naringin supplements (which are popular in weight loss compounds, apparently) isn’t as effective as drinking the actual juice for its intended effects.

This study indicates that it might not be the naringin at all; it might just be the whole grapefruit juice package. Here they found that naringin didn’t seriously affect a lot of the outcomes that come with caffeine, such as a raised resting energy expenditure (REE). In short, this study shows that naringin has little influence on the weight loss benefits of caffeine.

Does Grapefruit affect Caffeine?

When I started this article, I came in with the belief that it does, in fact, affect the effects of caffeine.

However, this study was unable to find any influence on several biological effects. They looked at several measures of blood pressure and heart rate. Their concluding statement was:

“We conclude that grapefruit juice had no effect on caffeine pharmacokinetics or hemodynamic effects.” (pharmacokinetics: the movement of the drug; hemodynamic: blood flow and transmission)

This study was the only one I could find that showed a marginal difference from grapefruit juice. They stated:

“[…] grapefruit juice decreased the oral clearance of caffeine by 23% and prolonged its half-life by 31%. […] However, the small effect on caffeine clearance in vivo suggests that in general the ingestion of grapefruit juice should not cause clinically significant inhibition of the metabolism of other drugs that are substrates of CYPIA2.”

In short, it had an effect, but not enough to be considered clinically significant when doctors suggest drugs that act on the same enzyme (CYPIA2).


Grapefruit has a compound, naringin, that slows the effects of several enzymes. It can affect several medications, including but not limited to: birth control (estrogen), sedatives, calcium channel blockers, blood pressure medications, allergies, AIDS, and cholesterol drugs.

But not caffeine. Not really. If grapefruits do, they don’t appear to do it in a way that’s easily measurable in a real way.

If you’re looking for a stronger pick-me-up, consider a red-eye or a double espresso.

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