ObjectivePremenstrual syndrome (PMS) affects millions of women and is known as the most important disorder among them. The very aim of the present study was to evaluate the effects of low dose calcium on severity of PMS.MethodsThis study can be considered as a double-blind randomized clinical trial. Female students of Hamadan University of Medical Sciences diagnosed with PMS in 2014 participated in the present study. Sixty-six female students diagnosed with PMS were involved in the experimental and control groups. The participants were randomly assigned into two groups to receive 500 mg of calcium daily or placebo for two months. Severity of PMS was detected by Daily Record of Severity of Problems, which was used to measure symptoms during one menstrual cycle before and two menstrual cycles after the intervention.ResultsNo signifcant differences were observed in the mean scores of PMS symptoms between calcium and placebo groups before the treatment (P=0.74). However, signifcant differences were noticed between the two intervention groups in the first (P=0.01) and second menstrual cycles (P=0.001) after the intervention. The differences were significant in subgroups of anxiety, depression, emotional changes, water retention, and somatic changes in calcium group compared with placebo group in the menstrual cycle before the intervention and two menstrual cycles after the intervention and among menstrual cycles (0, cycle 1, cycle 2) in calcium group (P=0.01).ConclusionOverall, the results of the present study suggest that treatment with calcium supplements is an effective method for reducing mood disorders during PMS.