Lotronex (alosetron hydrochloride) is a 5-HT3 receptor antagonist indicated for the treatment of irritable bowel syndrome (IBS) in females whose predominant bowel habit is diarrhea. Alosetron is extensively metabolized by multiple cytochrome P450 (CYP) enzymes, including CYP 2C9 and 3A4. Alprazolam is a short-acting benzodiazepine commonly prescribed for the treatment of anxiety disorders and a potential comedication in patients with IBS. Alprazolam is extensively metabolized by CYP3A4. This clinical study was conducted to assess the potential for a metabolic drug interaction between these two CYP3A4 substrates. This was an open-label, randomized, two-period, crossover study in 12 healthy female and male volunteers to determine the effect of concomitant administration of alosetron at the recommended dose of 1 mg p.o. bid on the pharmacokinetics of alprazolam following a single oral 1 mg dose. The results showed no effect of alosetron on the pharmacokinetics of alprazolam. Mean alprazolam AUC was 210 and 202 ng.h/mL in the absence and the presence of alosetron, respectively. Therefore, alprazolam may be safely coadministered with alosetron without the need for dosage adjustment.