# How does grapefruit affect drug action? **URL:** \ **Category:** Factual Questions\ **Created:** [January 28, 2016, 1:58am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446 "2016-01-28T01:58:02Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Nefario](https://avatars.discourse-cdn.com/v4/letter/n/59ef9b/32.png) [@Nefario](https://boards.straightdope.com/u/Nefario)\ **Post date:** [January 28, 2016, 1:58am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446/1 "2016-01-28T01:58:02Z")
I used to have the impression that grapefruit speeds up the absorption of certain drugs. Couldn’t this effect be used as a benefit? More recent warnings are more/less the nanny-state: “THIS IS DANGEROUS!” :eek: The only OTC pain reliever that seems to do _anything_ for me is naproxen sodium; and only one common antihistamine gives me the effect I like. I’ve thought of experimenting with grapefruit juice and normal drug dosages, especially with aspirin, acetaminophen, and ibuprofen. _NOT_ all at once… :rolleyes: What’s the worst that could happen? ---
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [January 28, 2016, 2:38am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446/2 "2016-01-28T02:38:33Z")
Besides the possible dangers. Having a drug taken up too quickly may waste a portion of it’s benefits. Some drugs work on particular chemicals in your body. Attaching to them, modifying or blocking them. Those chemicals are produced over time. Absorbing all the medicine at once, will have it interact with all the chemicals present at that time. The excess that cannot find interaction may be excreted before more of it’s targets are created. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [January 28, 2016, 3:32am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446/3 "2016-01-28T03:32:50Z")
The nanny state? Seriously? Anyway, the problem with grapefruit is not that it causes some drugs to be absorbed too fast. It’s the opposite problem. Grapefruits contain compounds called furanocoumarins which counteract an enzyme called cytochrome P450 3A4. And CYP3A4 (as it’s known to its friends) is the enzyme that breaks down many medications. So if the CYP3A4 isn’t working, the medications don’t break down. They either pass out of the patient’s body without any beneficial effect or accumulate in the patient’s body and possibly reach harmful concentrations. ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [January 28, 2016, 4:29am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446/4 "2016-01-28T04:29:21Z")
This was, in fact, just answered on [SciShow](https://www.youtube.com/watch?v=5Nxne8QfIhM) by Hank Green yesterday. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [January 28, 2016, 4:33am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446/5 "2016-01-28T04:33:25Z")
> [@Little\_Nemo](#): > > The nanny state? Seriously? > > Anyway, the problem with grapefruit is not that it causes some drugs to be absorbed too fast. It’s the opposite problem. Grapefruits contain compounds called furanocoumarins which counteract an enzyme called cytochrome P450 3A4. And CYP3A4 (as it’s known to its friends) is the enzyme that breaks down many medications. So if the CYP3A4 isn’t working, the medications don’t break down. They either pass out of the patient’s body without any beneficial effect or accumulate in the patient’s body and possibly reach harmful concentrations. I think this may not be completely correct. Grapefruit juice furancoumarins inhibit the CYP3A4 enzyme i_n the intestinal wall_. That means if a drug is normally broken down by the CYP3A4 enzyme in the intestinal wall, only a reduced quantity of intact drug (compared to what was ingested) is left to be absorbed. When grapefruit juice is present and inhibits the CYP3A4 enzyme, less drug is broken down in the intestinal wall and so more is absorbed than would have been anticipated. This is only a problem for drugs that are normally acted on to a large degree by intestinal CYP3A4. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [January 28, 2016, 4:38am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446/6 "2016-01-28T04:38:45Z")
Missed the edit window. [HERE](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1873672/)is an old (1998) review of the subject by the group that discovered the phenomenon. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [January 28, 2016, 7:49am UTC](https://boards.straightdope.com/t/how-does-grapefruit-affect-drug-action/744446/7 "2016-01-28T07:49:03Z")
> [@KarlGauss](#): > > When grapefruit juice is present and inhibits the CYP3A4 enzyme, less drug is broken down in the intestinal wall and so more is absorbed than would have been anticipated. Yes for many of the pharmaceuticals grapefruit causes overdose if much was expected to be left in the intestines,and the result is the patient is underdosed. There are some pharmaceuticals for which the case is other way around … CYP3A4 is actually required to turn the ingested substance into a metabolite …(and possibly the result in turn is converted later on to different metabolites… ) and so with the grapefruit inhibiting CYP3A4, the ingested substance is wasted, and the patient is underdosed.