Impotence drugs such as Viagra may do more than help men to have sex. They may also boost levels of a hormone linked with feelings of love, US researchers reported on Thursday.

Viagra (sildenafil) raised levels of the hormone oxytocin in rats, the team at the University of Wisconsin-Madison reported in theJournal of Physiology. This hormone is involved in nursing and childbirth, and also in orgasm and feelings of sexual pleasure.

It appears that Viagra and related drugs act on the part of the brain that controls release of oxytocin, said Wisconsin physiology professor Meyer Jackson.

"This is one piece in a puzzle in which many pieces are still not available," Jackson said in a statement. "But it raises the possibility that erectile dysfunction drugs could be doing more than just affecting erectile dysfunction."

Viagra, made by Pfizer Inc., is an inhibitor of an enzyme called phosphodiesterase type five. Related drugs such as Eli Lilly and Co.'s Cialis , known generically as tadalafil, and Levitra or vardenafil, sold by GlaxoSmithKline, Bayer AG and Schering-Plough, are also PDE-5 inhibitors.