Researchers at the University of Wisconsin-Madison report this month that sildenafil increases the amount of oxytocin released by stimulation of the posterior pituitary gland, a small structure directly underneath the brain that regulates hormone levels in response to neural signals.

The finding is the first indication of a chemical mechanism through which erectile dysfunction drugs like Viagra may have physical effects besides increasing blood flow to sexual organs, says study author Meyer Jackson, a physiology professor at the UW-Madison School of Medicine and Public Health.

Sometimes called the "love hormone" or "cuddle chemical," oxytocin plays several important roles in social interactions and reproduction, including triggering uterine contractions and lactation. It is also released during orgasm and has been linked to sexual arousal.

Oxytocin release is regulated by an enzyme that acts like a braking system, limiting hormone release by dampening neural excitation of the cells. This same enzyme, phosphodiesterase type 5, also limits blood flow by contracting the muscles around blood vessels.

In both places, sildenafil works by blocking this enzyme, essentially releasing the brakes, explains Jackson. In blood vessels, relaxing smooth muscle increases blood flow, which corrects erectile dysfunction, and in the posterior pituitary, the cells become more responsive. "The same stimulation will produce more [oxytocin] release."