An important difference between leuprolide and ganirelix is that ganirelix:

GnRH agonists like leuprolide, goserelin and nafarelin etc. are used by parenteral route. Continuous administration of these agents decreases gonadotropin secretion whereas pulsatile administration increases the secretion. When used continuously even then first few doses cause increased secretion of gonadotropins (LH and FSH) leading to flare up reaction in prostatic carcinoma. GnRH antagonists like cetrorelix and ganirelix are also used by parenteral route but these drugs immediately reduce gonadotropin secretion.