Objectives: The present study was carried out to investigate the relationship between different lunar phases and the occurrence of overdose attempted suicide. Method: The study was carried out for a period of three lunar years (Shaban 1416 to Shaban 1419) with patients admitted to the Intoxication Emergency Clinic at Imam Reza Hospital in Mashad. In this research, 13422 cases of suicide attempts, which were recorded in the hospital files were studied. Admission dates which were originally recorded according to solar calendar, were later converted to lunar calender for the purpose of this study. Then, on the basis of lunar phases and the shape of the moon as observed from the earth, the days of lunar months were divided into four periods: the crescent-to-half moon period, the half moon-to-full moon period, the full moon to half-moon period, and the half moon-to-crescent period. The data collected for each period were analyzed through chi2. Findings: The result demonstrated that the highest numbers of overdose suicide attempts were made during the full moon period and the days around it. The frequency of attempts shows a significant difference in the above periods (P