Wikipedia
Grapefruit contains a substance called furanocoumarin which can be found in some other fruits as well.
WebMD
The article also mentions Seville oranges a a potential hazard.
U.S. Food and Drug Administration (FDA)
A more complete list of foods and other substances known to interact with other substances.
I was also consuming oranges, kiwis, and bananas. I thought these were all good for me until I discovered Oxalate. Now I suspect Uric Acid is not the only acid that develops crystals inside your body, but the data on Oxalate suggests that it only accumulates in the kidneys.
I'm going to switch from oranges and kiwis to grapefruit, but I worry about the effects of furanocoumarin interacting with my over-the-counter pain meds.