21. Consumption of any of the following drugs that may interfere with the metabolism of 13C-Methacetin in the 48 hours prior to study enrollment including: Acyclovir, allopurinol, carbamazepine, cimetidine, ciprofloxacin, daidzein, disulfiram, Echinacea, enoxacin, famotidine, fluvoxamine, methoxsalen, mexiletine, montelukast, norfloxacin, phenylpropanolamine, phenytoin, propafenone, rifampin, terbinafine, ticlopidine, thiabendazole, verapamil, zileuton or oral contraceptives