C. GnRH Analogs: Agonists and Antagonists
Leuprolide acetate (Lupron®) is a potent agonist of GnRH, the hormone which controls the release of FSH and LH by the pituitary gland. It is usually taken as a daily subcutaneous injection. Its primary role is to prevent a premature LH surge, which could result in the release of eggs before they are ready to be retrieved. Since GnRH-agonists initially cause a release of FSH and LH from the pituitary, they can also be used to start the growth of the follicles or initiate the final stages of egg maturation (see Flare Protocol). 

Though leuprolide acetate is an FDA (Federal Drug Administration) approved medication, it has not been approved for use in IVF, although it has routinely been used in this way for more than 20 years.  Potential side effects usually experienced primarily with use for over 6 weeks include but are not limited to hot flashes, vaginal dryness, bone loss, nausea, vomiting, skin reactions at the injection site, fluid retention, muscle aches, headaches, and depression.  Few side effects are experienced with short-term use as is usually the case in IVF.

Ganirelix and cetrorelix (Antagon®, Cetrotide®) are antagonists of GnRH hormone and are also used to prevent premature ovulation.  As they have a rapid onset of action, they tend to be started later during the stimulation and used for just a few days. They are given subcutaneously. The potential side effects include, but are not limited to, abdominal pain, headaches, skin reaction at the injection site, and nausea.