Use of cimetidine to enhance systemic acyclovir concentrations in patients with ineffective suppressive therapy for recurring herpes simplex virus infections: A novel purpose for an old drug.

Herpes simplex virus (HSV) may cause recurring oral or genital ulcers. We report a series of patients nonresponsive to suppressive valacyclovir therapy, explained by subtherapeutic acyclovir plasma levels. After a dose increase, or in some patients only after concomitant prescription of cimetidine, adequate levels were reached associated with significant clinical improvement.