Low doses of anti-depressants like fluoxetine (Prozac) can help in preventing symptoms of premenstrual syndrome (PMS), according to scientists from the University of Bristol, UCL and the University of Sao Paolo-Ribeiro Preto in Brazil.

Up to 80 percent of women suffer from PMS, with symptoms such as anxiety, irritability, fatigue, sleep deprivation and increased pain sensitivity. Towards the end of the menstrual cycle there is decreased secretion of the ovarian sex steroid hormone progesterone. Decline in progesterone leads to decline in its breakdown product allopregnanolone, which acts in the brain as a potent sedative and tranquilizing agent. This appears to trigger PMS.

Mice studies have shown that short term treatment with low doses of the anti-depressants such as fluoxetine can inhibit a specific enzyme in the brain, which deactivates allopregnanolone, therefore maintaining the chemical balance of this in-built tranquilizer in the brain, and could be used to alleviate symptoms of progesterone withdrawal such as PMS and possibly also postnatal depression. It also blocked the increase in excitability of brain circuits involved in mediating the stress and fear responses that normally occur during this phase of the cycle.