Don't inadvertently mistake the grapefruit juice carton for the milk and put it in your coffee, wander back to your office, and take a big gulp.
As they say in tasting circles, citrus notes.
Grapefruit juice (all citrus, there's just generally more in grapefruit) contains a category of chemicals called psoralens which inhibit a key form of cytochrome p450 (an enzyme, mostly in the liver) that is involved in drug metabolism, so those drugs normally broken down by this cytochrome build up in the system and have a much longer half-life.