Grapefruits: Pharmacologically Evil

I’ve always tried to avoid paying attention to popular health articles that say things like: “Potatoes may decrease osteoporosis risk!” or “Potatoes may increase cataracts risk!” or “All leafy green things cure cancer except for mustard greens, which are acidic, and we all know that acid is terrible for you.” I mean, for every article like this (which, if you’re lucky enough to find a citation on what study they’re talking about, probably didn’t even come close to the headline’s conclusion), there’s another article that says the exact opposite. The obnoxiousness of the contradictions are popular reasons for people to view the whole subject of “good/bad vegetables” with suspicion.

So I was pretty surprised to find that half of our Pharmacology lecture this morning was dedicated to proving me wrong.

Drugs in our bodies are metabolized by cytochrome p450 enzymes, and these enzymes can be induced or inhibited by various other things you consume – whether they’re other drugs, air pollution, or foods you eat.  If you “induce” the right variant of the p450 enzyme (whichever one is responsible for a medication you take), you can decrease the concentration of that medication in your blood.  If you inhibit it, you can increase the concentration (and the risk of overdose).*

Interesting.  Apparently, cigarette smoke is a huge inducer of one of these enzymes, which struck me as strange (since it should be a good thing to clear cigarette smoke and car exhaust more rapidly from your body), but, since the world is out to get us, it turns out that the metabolites of cigarette smoke are more dangerous than the smoke itself.

So, fine.  “Cigarette smoking = bad” means the world makes sense again.  But what kills me about that is – you know what else induces that enzyme, presumably causing the same effects?  Broccoli and brussel sprouts. Nature has a sick sense of humor.

And an inhibitor of a p450 enzyme, strangely enough, is grapefruit juice.  This might be old news to you guys reading this, but I was shocked.**  It goes beyond the whole “well, grapefruit juice is an acid and changes the pH of the environment the drug is absorbed in” thing, which always made sense to me – it actually induces the darned enzyme and interferes with drug absorption like crazy by increasing the amount of the drug present in the blood.

I realize that none of this matters if you’re not on a relevant medication, and furthermore, that drug companies are required to tell you in the package inserts to not drink grapefruit juice.

… This is gonna sound dumb, but I just didn’t realize they meant it.

* This is all assuming that the drug isn’t a pro-drug, in which case things would be switched around, but still bad.
** One lecture is not exactly an adequate fund of knowledge to be acting like I know what I’m talking about.  Since this summarizes as “read your drug inserts and listen to your doctor”, I figure there’s no harm if I missed something – but let me know if that’s the case and I’ll edit this post.
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3 thoughts on “Grapefruits: Pharmacologically Evil

  1. No, actually you are lucky to get to see the package inserts on medications. I was actually on lisinopril for two years before a pharmacist warned me not to drink grapefruit juice.

  2. It might not be as clear-cut as the CYA package inserts say. When I was given a prescription for a med that says “no grapefruit,” I explained to my doctor that I’d rather skip the med and be able to eat grapefruit. He responded that it’s not really a problem; you’d have to eat way more grapefruit than is realistic. If I just eat one, it’ll be fine.

    • I believe it. That’s the trouble with CYA… you want it all covered.

      But now you’ve got me curious. I think I’ll ask the pharm professor how common it would be to be on one of these drugs and have a doseage problem from just 1 or 2 grapefruit.

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