Introduction : Premenstrual syndrome (PMS) encompasses a wide variety of cyclic and recurrent physical and emotional symptoms that occur before menstruation and has negative impact on activities of daily living and quality of life. The aim of this study is to investigate the effect of Omega-3 on the severity of physical and mood symptoms in premenstrual syndrome.

  Methods: This is a double-blind clinical trial. It is carried out on 100 female students which were randomly divided into two groups including omega-3 group and control (placebo) group. Every student took the drug during three subsequent menstural cycles. They took capsules daily in the first cycle for one month, and in the second and third cycles from eight days before menstruation to two days after that. They recorded the severity of their symptoms in a questionnaire. Data analyses was performed by SPSS version 17, using t-test.

  Results: Data showed that there were no significant differences between the two groups before intervention but 1, 2 and 3 months after taking Omega-3 capsules, the severity of physical and mood symptoms was significantly lower than than control group (p<0.001).

  Conclusion: Omega-3 significantly reduced the severity of physical and mood symptoms in premenstrual syndrome.