Italian scientists have determined that the changes to body chemistry which causes people to find new partners sexually attractive lasts, at most, two years.
At that point, couples shift into a "stable relationship" period, and other hormones dominate.
Hormones called neutrophins were present in much greater quantities in the blood of those in the early stages of a relationship. Testosterone also increased in women and decreased in men during this period.
This changed in those who had been in a relationship between one and two years; the "lust molecule" neutrophins were replaced by oxytocin, the so-called "cuddle hormone".
Some researchers were concerned that people might feel obligated to take hormone supplements in order to re-experience the initial rush of lust.