What is it about grapefruit specifically that messes with pretty much every prescription in existence?
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Pharmacist here. Grapefruit produces some compounds which are metabolized (broken down) by a family of enzymes called CYP3A4 (cytochrome p450 3A4). If my memory serves me right, the compounds are called coumarins which are related in structure to the blood thinner warfarin. Basically if someone has enough grapefruit or it’s juices, the body’s CYP3A4 enzymes will now have to spend time breaking the coumarins down in addition to whatever drug also needs that enzyme to be degraded. Most drugs have both major and minor enzymes involved in their degradation, but the minor enzymes typically will take much longer to break down the drug as they are typically less efficient and break down the drug in less than an optimal fashion. This results in the drug of choice taking much longer to get broken down and removed from the body, resulting in a longer half-life.This means the amount of time the drug will carry out its effects on its target will be increased, sometimes 5-6x or even more (I can’t remember the specific drug, but do remember a drug with a 4-6 hour half life taking more than 5-6 days to leave the body because it’s metabolism was so dependent on the 3A4 complex with the minor enzyme because less than 10% as efficient as 3A4). To be fair it’s not every drug in existence (although I realize OP is speaking hyperbole), but it is a fair number of them – my guess is around 25-30% of drugs will rely on CYP3A4. Some of these will be highly effected while others just marginally slowed.
Edit: TL;DR – imagine leaving a city at 3am… you have no traffic to contend with. This is your 3A4 system without grapefruits compounds competing with whatever drug youve take that’s dependent on the 3A4 system for metabolism. Now imagine leaving the city at 430pm during rush hour traffic. This is akin to the drug of interest competing with the grapefruits compounds for enzymatic degradation.
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