ROME (Reuters) - Your heartbeat accelerates, you have butterflies in the stomach, you feel euphoric and a bit silly. It's all part of falling passionately in love -- and scientists now tell us the feeling won't last more than a year.

The powerful emotions that bowl over new lovers are triggered by a molecule known as nerve growth factor (NGF), according to Pavia University researchers.

The Italian scientists found far higher levels of NGF in the blood of 58 people who had recently fallen madly in love than in that of a group of singles and people in long-term relationships.

But after a year with the same lover, the quantity of the 'love molecule' in their blood had fallen to the same level as that of the other groups.

The Italian researchers, publishing their study in the journal Psychoneuroendocrinology, said it was not clear how falling in love triggers higher levels of NGF, but the molecule clearly has an important role in the "social chemistry" between people at the start of a relationship.