Conversely, grapefruit juice can also reduce the quantity required of some drugs — and thus improve the efficacy, while reducing side effects. In some cases, that can be a big money-saver (less of an expensive drug required).
I recall in some Hunter S. Thompson book (Fear & Loathing in Las Vegas???) where he writes about several hookers, coke and two crates of Grapefruit on his list of expenses for a Rolling Stones article.
http://www.extracrispy.com/culture/456/eating-breakfast-like-hunter-s-thompson-is-a-terrible-idea