I remember a couple of a years ago when my pdoc warned me about my intake of grapefruit products in combination with Xanax.

Turns out that there is a list of meds that grapefruit can interfere with-- I think it interacts with an enzyme in the intenstine. It can increase the levels of med in the bloodstream, taking it from a therapeutic doseage to just over-- thus, toxic.

I imagine this fact is little known-- my previous pdoc was the only one who ever mentioned it to me-- and he only specified it in regards to Xanax... I didn't know it could interact with other meds until I looked it up myself.