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Quercetin, one of the most abundant natural flavonoids, has been reported to modulate the activity of several drug‐metabolising enzymes. The aim of the present study was to investigate the effects of quercetin on cytochrome P450 (CYP) 1A2, CYP2A6, N‐acetyltransferase (NAT2) and xanthine oxidase (XO) activity in healthy volunteers using caffeine as a probe drug.

Twelve unrelated, healthy volunteers were recruited to the study. There were two phases to the study; in the first phase, each subject was given a single oral dose of caffeine (one 100 mg capsule) with 150 mL water; in the second phase, each subject was give a 500 mg quercetin capsule once daily for 13 continuous days and was coadministered a 100 mg caffeine capsule on the 13th day. Urinary caffeine metabolite ratios were used as indicators of the activity of CYP1A2, CYP2A6, NAT2 and XO. The pharmacokinetics of caffeine and its …