What is it about grapefruit specifically that messes with pretty much every prescription in existence?
In:
Intro: Most oral medications need to enter the bloodstream at a reliable concentration to work, something not too high for your body (causes toxicity) and not too low (causes loss of efficacy). There’s a lot biochemical players involved in shuffling drugs towards the bloodstream, a handful of which vary a ton between different individuals and lifestyles.
Answer: One compound in grapefruit juice IRREVERSIBLY inhibits a primary drug metabolizing enzyme within your intestines, CYP3A4. The enzyme normally breaks down drugs before they can enter the bloodstream, at a regular/predictable pace that can be accounted for during drug development. But if the enzyme suddenly gets knocked out, affected drugs cross into the bloodstream unchecked and cause some degree of overdosing. At least, until your body can produce a fresh batch of enzymes to overwrite that glass of grapefruit juice.
PS: It doesn’t even end here. Oral drugs also recirculate through your body. More and more of it should get broken down by enzymes and excreted from your body with each pass. The drug cross from the intestines into the bloodstream, from the bloodstream into the liver, and from the liver back into the GI tract — where it might encounter the impaired enzyme yet again. If someone keeps chugging grapefruit juice, the drug hangs around longer than intended and accumulates with each scheduled drug dose.
Related: Some other drugs (like Allegra) can only enter the bloodstream through a transporter channel in the intestines. Grapefruit juice also seems to inhibit this transporter, which can lead to drug underdosing and loss of efficacy.
LPT: Drugs can have potent interactions not only with other drugs, but also with different botanical foods and supplements — grapefruit juice happens to be super well documented, along with St. John’s Wort. Always check with your pharmacist to see what’s right for you.
[https://www.fda.gov/consumers/consumer-updates/grapefruit-juice-and-some-drugs-dont-mix](https://www.fda.gov/consumers/consumer-updates/grapefruit-juice-and-some-drugs-dont-mix)
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