Endogenous gonadotropin release may also be prevented by using GnRH antagonists instead of GnRH agonists. During the past decade, two GnRH antagonists (cetrorelix, ganirelix) have been licensed for prevention of premature LH surge and ovulation during ovarian hyperstimulation for assisted reproduction. The antagonist competes directly with the physiological GnRH for binding to the pituitary GnRH receptors and provides quicker suppression of gonadotropin release without the initial flare up. Compared with the GnRH agonist long protocol, GnRH antagonists require a considerably shorter treatment period, less exogenous gonadotropin for ovarian stimulation, and are associated with fewer side effects, and a lower risk of ovarian hyperstimulation syndrome (OHSS). An early meta-analysis from 2002, which included the first studies comparing GnRH agonist and antagonist treatment regimens, showed that clinical pregnancy was lower with GnRH antagonist treatment. However, there were no statistically significant differences in life-birth rate or in the probability of life birth between the protocols, as shown in a more recent review. However, GnRH agonists are still routinely used in most infertility centres, whereas GnRH antagonists are still preferentially prescribed to older women with unfavourable prognosis. The equivalence of both protocols has only been evaluated in ovarian hyperstimulation based on recombinant FSH only.