The effect of viloxazine on the pharmacokinetics of theophylline was studied in eight healthy volunteers. Theophylline 200 mg/day (théophylline Bruneau 100 mg tablets) was administered on day 1; after a 3-day washout period, viloxazine 300 mg/day (Vivalan 100 mg tablets) was administered orally from days 5 to 7. On day 8, theophylline 200 mg and viloxazine 100 mg were concomitantly administered. The pharmacokinetic parameters of theophylline alone and after coadministration of viloxazine were determined. Viloxazine significantly increased the plasma concentrations (p < 0.01) and the areaunder-the-curve values (p < 0.01) of theophylline and decreased its body clearance (p < 0.05). Our results suggest that the dosage of theophylline should be decreased and its plasma concentrations monitored when viloxazine is prescribed.