This is 5-geranoxypsoralen. It’s found in many citrus fruits, but at an especially high concentration in grapefruit.
When you take some medications, or eat food, or drink $12 fresh-pressed juices, or lick hallucinogenic frogs, etc., an enzyme called cytochrome P450 alters some of the foreign molecules you ingested so they are more easily cleared by your body (AKA pee it out).
This is one of the first steps in your body’s built-in detox system (so yes, your body detoxes the detox juice you just drank 😉). 5-geranoxypsoralen, however, inhbits cytochrome P450 (specifically CYP3A4), which means that if you drink grapefruit juice, the enzyme cannot modify those molecules to clear them. This is usually fine unless you are taking certain medications.
When you are prescribed a medicine, the dose takes into account the amount that will be lost by cytochrome P450. But, if cytochrome P450 is knocked out by the grapefruit juice, it will increase the effective amount of medicine your body will get, sometimes with toxic side effects.
So, PSA, always check your medications’ labels for warnings about citrus juice. Common contraindicated drugs are Lipitor (atorvastatin), Buspar (buspirone), and Uceris (budesonide).
Source: Acta Pharmacol Sin. 2004 Feb;25(2):129-36