This is summarised in a 2020 systematic review, which found that metformin may reduce the incidence of OHSS by more than half when compared with placebo or no treatment in patients undergoing a long GnRH agonist protocol (relative risk (RR) 0.40, 95% confidence interval (CI) 0.26 to 0.60) (37). This evidence concerns the gene GNRH1 and ovarian hyperstimulation syndrome.