The hormone oxytocin has a reputation so warm and fuzzy that it's sometimes called the love hormone. It plays a pivotal role in mother-infant bonding and can make introverts feel more outgoing. It's even being studied as a treatment for autism, anxiety and schizophrenia.

But these efforts began without much prior research on the long-term effects of oxytocin. And now results from recent animal studies suggest a darker side to the hormone.

Oxytocin's long-term effects appear to be quite different from its single-dose effects of love and good cheer.

For example, oxytocin can strengthen negative memories of a stressful event in mice so much that these memories remain long after the stressful event is over. These memories can then add to the fear and anxiety felt in future stressful situations, much like post-traumatic stress disorder (PTSD), making a bad situation worse.

A new study found that oxytocin's long-term effects appear to be quite different from its single-dose effects of love and good cheer. Working with the prairie vole, a small rodent that mates for life, researchers noted that at doses currently being used in human trials, long-term oxytocin use disrupts normal male behavior.

Prairie voles are often used in studies of social behavior because, unlike most animals, they are monogamous. In this study, voles were either given one of three different doses of nasal oxytocin spray or a dose of saline daily, from the day they were weaned until they reached sexual maturity.

Initially, oxytocin increased social behavior in the males, similar to the short-term effects seen in humans. But as the oxytocin-treated males got older, their behavior became abnormal.

“Male prairie voles which received a dose similar to that being tested in humans, or even a lower dose, did not form pair-bonds normally with their pair-mate. Instead these males chose to associate with a strange female,” said Dr. Karen Bales, Professor and Vice Chair of Psychology at the University of California, Davis, in a press release.

Some might say that this is typical behavior for a human male, but it's decidedly aberrant for a prairie vole, where males rarely look for a new partner even if their mate dies. And in one human study, oxytocin's short-term effect on men who already had a partner was to keep them further away than normal from strange women, not to encourage their advances.

If oxytocin's long-term effects are so different from its short-term impact, this poses great problems for its long-term use for any human condition. The findings suggest that oxytocin treatments caused long-term changes in the oxytocin system, according to Dr. Bales.

Oxytocin research is still in its infancy and the prairie vole study is unlikely to be the final word on its usefulness. But it does point out that there's more to oxytocin than warmth and fuzziness.

The study appears in Biological Psychiatry .