system
Given the following post, respond as if you're commenting on Reddit:
human
What is it about grapefruit that has some medicines say "Do not eat grapefruit or drink grapefruit juice while on this medicine"?

I was recently given some samples of Seroquel XR by my doctor, and it has a grapefruit warning on it.

My dad takes several medicines for his heart and I believe several of them have the "no grapefruit" warning on them too.

What is it about grapefruit that isn't in other citruses?
gpt
This is a great question and a very important one. Grapefruit juice (and some other fruit juices, but to a much lower extent) contain molecules, such as bergamottin, dihydroxybergamottin, and paradicin-A, which inhibit the metabolism of drugs that are metabolized by a specific liver enzyme (CYP3A4). Drug dosages are carefully calculated so that you have a specific concentration of the drug in your blood (plasma concentration). This concentration is affected by the rate of removal of that drug from the blood (metabolism is one way in which drugs are eliminated). When these molecules in grapefruit juice are processed by the liver, it reduces the metabolism of the drug. When the rate of elimination of the drug is decreased, it will reach a higher plasma concentration that predicted for a given dose. This can result in an overdose.