Fluctuations in female sex hormones may make women more apt to use drugs, have sex, or engage in other pleasurable activities at certain times of the month, researchers say.

The Telegraph reported Feb. 5 that researchers from the National Institute of Mental Health found that changes in estrogen levels make the brain's reward system more responsive during the first part of the menstrual cycle. "Increased activity of the brain's reward system at this time could boost anticipation and enjoyment of sexual activity," said researcher Karen Berman. "This demonstrates for the first time that female hormones affect the reward system in very specific ways during particular parts of the cycle."

Previous studies have shown that female users of cocaine and amphetamines appear to get more pleasure from taking these drugs during the early part of their fertility cycle.

The study was published Jan. 31, 2007 in the Proceedings of the National Academy of Sciences.