Regrading grapefruit juice interactions with medications we take, now more than a decade has passed since it was discovered that grapefruit juice interacts with drugs. Here is the gist of this paper as it applies to our everyday lives, taken verbatim from the article itself.
“Grapefruit juice does not need to be taken simultaneously with the medication, in order to produce the interaction. The bioavailability of lovastatin has been reported to be doubled, even when taken 12 h after the juice intake (Rogers et al, 1999), and the effects of grapefruit juice on felodipine have been shown to exist at about 30% of its maximum even when the drug was taken 24h after the juice intake (Lundahl et al, 1995). The prolongation of the interaction is consistent with the pharmacological mechanism of the interaction, since biosynthesis of a new enzyme is necessary. Up to 3 days persistence of the juice impact is reported (Takanaga et al, 2000); however, it appears that an interval of 24h between ingestion of grapefruit juice and a drug can usually prevent a potential clinically relevant interaction (Lilja et al, 2000b).”
Research Paper: http://www.nature.com/ejcn/journal/v58/n1/pdf/1601736a.pdf