I haven't researched this before, but I find it interesting that the warnings in this post make me wonder about the stuff Tim Ferriss was recommending in The Four Hour Body (specifically grapefruit juice + coffee). I wonder if anyone has had issues following some of the advice in that book.
There shouldn't be any issues unless one is doing grapefruit AND taking pharmaceuticals (which hopefully the majorit of Tim's readers are not). Statins are probably the big one for one section of the Ferris demographic (middle aged men). But A. I'm guessing most people don't eat grapefruit or grapefruit juice (it's not that common) and B. if they do it's not much. Who drinks a glass of grapefruit juice every day? I'm guessing, hardly anyone.
Ironically, probably the majority of the people that do drink a glass of grapefruit juice every day are of the age that they're likely to be on one or more pharmaceuticals.