They interact with so many drugs, if you take any medication for chronic illnesses, chances are you should not eat any grapefruit for the rest of your life. Also whether a drug is affected by it or not is not clear, so the common advice is just to avoid the fruit whatever the drug you're taking.
It's one of my favourite fruits, and not being able to eat it once in a while hurts. Though I'm not sure how _dangerous_ it really is. Does it affect the half-life of a drug by 5%, or by 100%?
Maybe someone with journal access can dig out dosage ranges from any of these many sources: https://scholar.google.com/scholar?q=related:4ssWhTcQeYEJ:sc...
(Source: have accidentally double dosed my amphetamine Rx. A 5x dose would probably be lethal.)
I know of somebody whose boyfriend ended things with her, she took a load of tylenol to end her life, but the doctors managed to resuscitate her. When she came to, she said to her mother "I'm so glad I didn't kill myself over that bastard". A week later she died from liver failure.
At that point it's either a transplant or death.
The scary thing is that it's incredibly easy to overdose by accident. For some people the lethal dose is less than twice the recommended dose, and if you're in a lot of pain - e.g. from a dental abscess - you can easily overdo it.
On a pedantic note, acetaminophen should not be considered innocent, but rather considered to be something very easy to get too much of because it’s in friggin’ everything. Cough syrup, Tylenol, many opioids that your doctor might prescribe. Read those labels, because if you take some Tylenol for those aches and pains and wash it down with some cough syrup for that cold, your liver might want to have a word with you.