IF you always thought you had a special chemistry with your loved one, you may finally have been proved right.

Researchers have found that a passionate kiss unleashes a complex chemical surge into the brain which makes a lover feel excited, happy or relaxed.

There is also speculation that this hormone release may be triggered directly by an exchange of sexually stimulating pheromones in the saliva.

“This study shows kissing is much more complex and causes hormonal changes and things we never thought occurred,” said Wendy Hill, professor of psychology at Lafayette College, Pennsylvania, in an interview.

“We tend to think more about who we are kissing and how it feels, yet there are a lot of other things happening.”

Scientists may have taken a while to catch up on kissing but others have been clear about its impact for centuries.

William Shakespeare described the effect in Romeo and Juliet where, after the couple kiss for the first time, Romeo says: “Sin from thy lips? O trespass sweetly urged! Give me my sin again.”

Hill wanted to find out just what happens to evoke such a powerful emotional response from simply rubbing lips. Her research looked at the impact of kissing on levels of two hormones, oxytocin and cortisol, in 15 male-female couples before and after holding hands and before and after kissing.

Oxytocin is known to be involved in social bonding so the researchers predicted that its levels would rise, while cortisol, a stress hormone, would fall. The results showed cortisol levels fell in both sexes, although oxytocin levels rose in men but fell in women.

This was an unexpected result but Hill and her co-researchers believe the fact that the tests were carried out in an unromantic campus health centre also played a part. Over the past year they have run the tests again in a softer setting complete with romantic background music.

Detailed results will be published at the American Association for the Advancement of Science’s annual conference in Chicago this week but are understood to confirm a close association between kissing and hormone levels.

Other scientists have been looking at the importance of the first kiss, an act long known to have the power to make or break a relationship.

In the 2005 film Hitch, Will Smith plays a New York matchmaker who helps men get dates. He says: “One look, one kiss, that’s all we get . . . to make the difference between happily ever after and, ‘Oh, he’s just some guy I went to some thing with once’.”

Such views are underlined by Professor Susan Hughes, a psychologist at Albright College, Pennsylvania, and co-author of Sex Differences in Romantic Kissing among College Students: An Evolutionary Perspective, whose research suggests that women use kissing as a way of screening potential lovers.

She said: “Females place a lot more importance on the breath and teeth of the person. This shows how well you care for yourself and your hygiene and women are a lot more picky when it comes to that.”

One puzzle is just how kissing might induce hormonal changes of the kind found by Hill. There are clearly psychological factors involved but some researchers suspect saliva contains pheromones, chemical messengers known to be important in other mammals.

In humans the role of pheromones is controversial because we lack organs to detect them. However, Sarah Woodley, an assistant professor at Duquesne University, Pittsburgh, who will speak at the same meeting, believes that people can still detect them via the nose.

Helen Fisher, professor of anthropology at Rutgers University, New Jersey, and author of Why We Love, believes that kissing produces not just a chemical, sexual thrill but can even improve overall health: “If you’re sharing your germs with somebody, you’re boosting your internal defence system.”

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