Couples should not worry when the first flush of passion dims - scientists have identified the hormone changes which cause the switch from lust to cuddles.
Certain hormones are active during the 'acute love' phase
A team from the University of Pisa in Italy found the bodily chemistry which makes people sexually attractive to new partners lasts, at most, two years.
When couples move into a "stable relationship" phase, other hormones take over, Chemistry World reports.
But one psychologist warned the hormone shift is wrongly seen as negative.
Dr Petra Boynton, of the British Psychological Society, said there was a danger people might feel they should take hormone supplements to make them feel the initial rush of lust once more.
The Italian researchers tested the levels of the hormones called neutrophins in the blood of volunteers who were rated on a passionate love scale.
Levels of these chemical messengers were much higher in those who were in the early stages of romance.
Testosterone was also found to increase in love-struck women, but to reduce in men when they are in love.
But in people who had been with their partners for between one and two years these so-called "love molecules" had gone, even though the relationship had survived.
The scientists found that the lust molecule was replaced by the so-called "cuddle hormone" - oxytocin - in couples who had been together for several years.
Oxytocin, is a chemical that induces labour and milk-production in new and pregnant mothers.
Donatella Marazziti, who led the research team, said: "If lovers swear their feelings to be ever-lasting, the hormones tell a different story."
Similar research conducted by Enzo Emanuele at the University of Pavia found that levels of a chemical messenger called nerve growth factor (NGF) increased with romantic intensity.
After one to two years, NGF levels had reduced to normal.
'Real Cupid's arrows'
The researchers said: "Whether more nerve growth is needed in the early stage of romance because of all the new experiences that are engraved into the brain, or whether it has a second, as yet unknown function in the chemistry of love, remains to be explored."
Michael Gross, a bio-chemist and science writer who has studied the latest findings, said: "It shows that different hormones are present in the blood when people are acutely in love while there is no evidence of the same hormones in people who have been in a stable relationship for many years.
"In fact the love molecules can disappear as early as 12 months after a relationship has started to be replaced by another chemical glue that keeps couples together."
He added: "To any romantically inclined chemist, it should be deeply satisfying to be able to prove that chemical messengers communicate romantic feeling between humans."
"It may be the only thing that science can offer as a real-world analogy to Cupid's arrows."
But Dr Boynton said: "This feeds into a 1970s view that when you meet it's all sparky, and then it's a downward trajectory to cuddles - which is seen as a negative.
"It is suggesting that what happens first is the best bit - and that isn't true."
She added: "I'm concerned that, having identified these hormones, there will be some move to suggest replacements to recreate the early passion."