The effects of sildenafil on human sperm function in healthy volunteers.

Aims: This double-blind, randomized, four-period, two-way crossover study was conducted to evaluate the acute effects of oral sildenafil (100-mg single dose) on sperm motility, count, density, morphology and vitality as well as ejaculate volume and viscosity in healthy male subjects. The concentrations of sildenafil and its primary circulating metabolite UK-103,320 were measured in ejaculate and compared with those in plasma. The study also included assessments of safety and tolerability.