WASHINGTON (Reuters) - Impotence drugs such as Viagra may do more than help men physically have sex -- they may also boost levels of a hormone linked with feelings of love, U.S. researchers reported on Thursday.

Viagra, known generically as sildenafil, raised levels of the hormone oxytocin in rats, the team at the University of Wisconsin-Madison said in a report published in the Journal of Physiology.

This hormone is involved in nursing and childbirth but also in orgasm and feelings of sexual pleasure.

And it seems 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 5. 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.

They block this enzyme, which in turn breaks down other compounds. This increases blood flow in the muscles but it also affects a brain structure known as the posterior pituitary.

This, in turn, boosts oxytocin, at least in the rats. It probably does the same thing in people, Jackson said.

"It does the same thing it does in smooth muscle -- instead of (levels) coming down in a minute or two, they stay up a little longer," Jackson said in a telephone interview.

Oxytocin was known for years to be involved in labor and it is the hormone that causes the "let-down" of milk in breastfeeding babies. Only in recent decades has it been found to have a function in men -- in sexual arousal and function.

This could suggest other uses for Viagra and related drugs, Jackson said -- perhaps promoting social bonding.

"What I hope is that people read our paper who can test these ideas in animals and humans," Jackson said.

"I hope that this doesn't cause some wild orgy of inappropriate recreational use."

Some groups have complained that people use the impotence drugs for fun, instead of using them as prescribed for sexual dysfunction caused by low blood flow to the genitals.

The drugs can cause fatal side effects if used with certain other drugs, and some studies also suggest they may affect the eyes in some patients, so doctors stress they are not for casual use.