Third-generation non-steroidal aromatase inhibitor
ATC code: L02BG04 (Letrozole)
Brand names
Femara, Letrobel, Loxifan
A selective reversible inhibitor of aromatase – the cytochrome P450 enzyme converting androgens (testosterone, androstenedione) to estrogens (estradiol, estrone) in peripheral tissues. In postmenopausal women, lowers plasma estradiol by 95–98%. Does not affect adrenal cortex or cortisol synthesis. Half-life 2 days; once-daily dosing.
First line
First-line adjuvant endocrine therapy for hormone-receptor positive breast cancer in postmenopausal women per ESMO 2024 and NCCN 2024. Dose 2.5 mg once daily for 5–10 years. The BIG 1-98 trial showed superiority over tamoxifen in disease-free survival. In metastatic ER+ HER2-negative disease, first-line combined with a CDK4/6 inhibitor (palbociclib, ribociclib, abemaciclib) or fulvestrant.
Postmenopausal women only. In premenopausal patients, concurrent ovarian function suppression is required.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
Letrozole is used by amateur bodybuilders and AAS users as a more potent aromatase inhibitor. ASCO 2022 and the AEMPS Ficha Técnica confine letrozole to hormone-receptor-positive breast cancer. Approved indications in male athletes do not exist. Use causes marked bone mineral density loss, arthralgia, hypercholesterolemia, psychiatric effects and cardiovascular risk.
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Absolute contraindication. Teratogen. If pregnancy occurs on therapy, immediate discontinuation and obstetric consultation are required.
Contraindicated during lactation.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
Letrozole is evaluated for the following indications with varying evidence strength: ER-positive breast cancer (evidence tier A), Athletic endurance (evidence tier F). See the full indication matrix with dosing and citations above on this page.
Common side effects of Letrozole (≥ 1 in 100): Hot flashes and night sweats, Joint and muscle pain, Fatigue, Nausea, Reduced libido and vaginal dryness, Weight gain. See the Safety section for uncommon and serious reactions.
FDA category X. Absolute contraindication. Teratogen. If pregnancy occurs on therapy, immediate discontinuation and obstetric consultation are required.
Contraindicated during lactation.
Letrozole is contraindicated in: Letrozole hypersensitivity; Premenopausal status without ovarian suppression; Pregnancy and lactation; Severe hepatic impairment. Full list in the Safety section.