A temporary increase in testosterone levels in the body caused by certain types of hormone therapy used to treat prostate cancer. A testosterone flare may occur in patients who are taking drugs called gonadotropin-releasing hormone (GnRH) agonists. When first given, these drugs cause the testicles to make more testosterone. This may cause symptoms, such as bone pain and urinary problems, to get worse, but they usually go away after the first few weeks of treatment.