What is it about grapefruit specifically that messes with pretty much every prescription in existence?
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I’m seeing a lot of partially correct answers, so I wanted to point out that there are multiple different mechanisms in which grapefruit can interfere with drug metabolism. All of them are caused by certain chemical compounds found in the citrus (called furanocoumarins). The three main ways they can do this are:
1. Inhibit metabolizing enzymes from breaking down the active form of the drug to the inactive form. This can lead to increased concentration of the drug in your body which can be toxic and lead to overdose (lots of drugs are affected this way, such as antidepressants and cholesterol medications)
2. Inhibit metabolizing enzymes from activating a prodrug to the active form of the drug. This can lead to decreased concentration of the functional drug in your body, reducing the drugs effectiveness. (Certain blood thinners are affected this way)
3. Blocks intestinal transport proteins from absorbing the drug from your digestive tract into your blood stream. This causes the drug to pass through your body without being absorbed, reducing the drug’s effectiveness (Allegra is affected in this way)
For mechanisms 1 and 2, if only small amounts of grapefruit is consumed, only the intestinal metabolic enzymes are affected, meaning that only drugs taken orally would be impacted. If lots of grapefruit is consumed, liver enzymes are also affected, which means that drugs taken via any administration method are impacted.
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