In the present study, which includes 287 participants, metformin has been used by women undergoing IVF/intracytoplasmic sperm injection for more than 5 weeks before and during treatment and during luteal phase. There was no significant difference in number of gonadotrophins used, days of stimulation, number of oocytes retrieved, and number of embryos replaced. There was no significant difference in clinical pregnancy rate between both groups but there was significant reduction in the incidence of ovarian hyperstimulation syndrome (OHSS) in the group taking metformin. Metformin is a safe, cheap drug that can help in prevention of OHSS.