GnRH Analogs (Leuprolide, Degarelix)

GnRH analogs are a classic USMLE Step 1 pharmacology topic built around one core paradox: a drug that mimics GnRH can actually suppress testosterone. That seems backwards until you understand receptor biology. The hypothalamus releases GnRH in pulses, which is what keeps LH and FSH secretion functional. When you give a GnRH agonist like leuprolide continuously, you flood the receptor — it downregulates, and the whole axis shuts down. The result is chemical castration, which is exactly what you want in hormone-sensitive prostate cancer or endometriosis.

The exam tests this at multiple levels. Pure recall questions ask about indications. Application questions give you a patient on leuprolide and ask why testosterone levels are now low despite using a 'stimulating' drug. Passage-based questions may describe a clinical scenario where a patient with bone mets gets worse after starting leuprolide — and you need to recognize the initial flare phenomenon and explain why an antiandrogen is co-administered. The tricky part is that students who memorize 'GnRH agonist suppresses testosterone' without understanding the mechanism will get the easy questions right but bomb the harder ones.

Degarelix adds a second layer: it's a GnRH receptor antagonist, not an agonist, so it blocks the receptor directly and avoids the initial flare entirely. Students often conflate the two drugs mechanistically, assuming both work by receptor downregulation. They don't. Keeping that distinction sharp is what separates high scorers from average ones on USMLE Step 1.