Aromatase inhibitors prevent the estrogen rise associated with the flare effect of gonadotropins in patients treated with GnRH agonists.

This was a preliminary study to determine the effect of aromatase inhibitors in preventing the flare induced by GnRH agonist (GnRH-a) in women with endometriosis (n = 9) or leiomyomata (n = 4) who were given letrozole on the first 5 days of the GnRH-a therapy. Serum LH and FSH levels showed the typical flare 1 day after the injection of the GnRH-a; however, E(2) declined immediately after letrozole administration and was notably suppressed at 1, 2, and 4 days after GnRH-a. No patients complained of clinical symptoms typical of the GnRH-a flare phenomenon.