Grapefruit juice (GFJ) has been found to interact with several medications, increasing their oral bioavailability and the risk of toxicity. Inhibition of CYP3A4 in the small intestine by flavonoids (such as naringin and naringenin) and furanocoumarins (including bergamottin and 6',7'-dihydroxybergamottin) present in GFJ seems to be the predominant mechanism, although P-glycoprotein and influx transporters in the small intestine are also involved. The quantity of interactive compounds ingested may affect the magnitude and mechanism of the food-drug interaction. Therefore, these four compounds were quantified by HPLC analysis in commercially available and fresh-squeezed GFJ and in grapefruit tissues. Considerable variability in naringin (174-1492 micromol/L), bergamottin (1.0-36.6 micromol/L), and 6',7'-dihydroxybergamottin (0.22-52.5 micromol/L) was observed, whereas naringenin could not be detected. White grapefruit showed higher concentrations of naringin and furanocoumarins located in the albedo and flavedo compared with red varieties. Findings from this study suggest considering concentrations of components with a potential for drug interactions in GFJ-drug interaction studies. The concentration of potentially contributing compounds may crucially influence the magnitude of observed interaction and impair direct comparison of studies in which different juices have been used.