Letrozole — Non-steroidal aromatase inhibitor (AI) — FDA-approved for breast cancer; most potent AI for estrogen suppression; used off-label in TRT for aggressive estrogen management

Letrozole is a third-generation non-steroidal aromatase inhibitor (like anastrozole), but significantly more potent. It competitively and reversibly inhibits the aromatase enzyme (CYP19A1) — the enzyme responsible for converting androgens (testosterone, androstenedione) to estrogens (estradiol, estrone). Potency comparison: letrozole suppresses plasma estradiol by ~98–99% in postmenopausal women; anastrozole suppresses by ~85–95%. The difference in potency has clinical implications: (1) In TRT: letrozole is more effective at reducing high estradiol, but the risk of OVER-suppression (estradiol crash) is substantially higher; (2) In male fertility protocols: sometimes used to increase testosterone:estradiol ratio and improve spermatogenesis; (3) In AAS use / PCT: occasionally used but risks same over-suppression. KEY DISTINCTION from anastrozole: letrozole is ~5–10 times more potent per milligram. The same absolute dose produces far more estrogen suppression. For most TRT patients managing estradiol, anastrozole is preferred for this reason — letrozole's potency makes precise estrogen management difficult.