Flare Effect (GnRH Agonists)
The initial stimulatory surge in sex hormones (testosterone or oestrogen) that occurs during the first 1-2 weeks of GnRH agonist therapy, before receptor downregulation produces the intended suppression. The flare can temporarily worsen symptoms and is avoided by using GnRH antagonists instead.
Technical Context
The GnRH agonist flare occurs because initial receptor binding stimulates gonadotroph signalling before desensitisation/downregulation develops. Duration: testosterone levels peak at approximately day 3-7 and typically return to baseline by day 14-21 before declining to castrate levels by day 28. Clinical significance: in prostate cancer, the testosterone flare can temporarily worsen bone pain, urinary obstruction, and spinal cord compression (from vertebral metastases) — potentially causing clinically dangerous disease flare. Mitigation strategies: pre-treatment or concurrent anti-androgen therapy (bicalutamide, flutamide) to block testosterone action during the flare period, or use of GnRH antagonists (degarelix, relugolix) which produce immediate suppression without flare. For endometriosis and fertility applications, the initial hormonal surge is generally less clinically dangerous.