Letrozole (Femara)
Triazole-class non-steroidal AI, stronger than anastrozole (~99% E2 suppression at 2.5 mg/day). FDA-approved (1997 Novartis). AAS-niche: hard-suppression Tren+Mast E2-control + gynecomastia reversal (2.5 mg/day × 2-3 weeks).
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WHAT IS LETROZOLE (FEMARA)?
Detailed overview
Letrozole (Femara) is a third-generation non-steroidal triazole AI developed by Novartis and FDA-approved in 1997 for postmenopausal ER+ breast cancer. A stronger aromatase inhibitor than anastrozole (~97%): the BIG1-98 trial (Coates 2007, PMID 17582920) documented ~99% serum E2 suppression at 2.5 mg/day – the head-to-head trial of the two AIs (Geisler 2002) showed letrozole has greater intramuscular aromatase tissue penetration (stronger peripheral aromatase action). In the AAS world there are two main uses: (1) E2-control on hard-suppression stacks (Tren-Acetate + Mast-Prop + Test-Prop blast – on heavily aromatizing steroids like Dianabol or Anadrol combinations), and (2) gynecomastia reversal (ER-mediated breast gland tissue shrinkage, 2.5 mg/day × 14-21 days with taper protocol). Because it's stronger than anastrozole, AI-naive users should start at anastrozole-equivalent half-doses. WADA S4.1 – banned.
Mechanism
Reversible non-steroidal CYP19 aromatase inhibitor, ~99% serum E2 suppression at 2.5 mg/day
Dosing (AAS)
0.25-1.25 mg EOD bloodwork-titrated; gyno reversal 2.5 mg/day × 14-21 days
Half-life
~42 hours
Onset
E2 reduction measurable 24-48 h, plateau 4-6 days
Legal status
FDA + EMA Rx, HU + PL approved, WADA S4.1 banned
Data console
Lab data
Safety
Side effects, stop signs, contraindications
Side effects · 7
- Over-suppressed (crashed) estradiol: letrozole is more potent than anastrozole (~99% E2 suppression), so it easily drives E2 into symptomatic hypoestrogenism, causing low libido, erectile dysfunction, fatigue and low mood.
- Joint and muscle pain (arthralgia, myalgia): a hallmark of estrogen deprivation, with wrist, knee and hand symptoms and joint stiffness; the most common complaint in clinical trials.
- Worsening lipid profile: with sustained use HDL falls (up to ~30%) and LDL rises (~15-25%), shifting the profile toward unfavourable cardiovascular risk, especially on an aromatizing steroid stack.
- Bone loss and osteoporosis: estrogen is key to maintaining bone density, so prolonged letrozole use lowers BMD and raises the risk of osteoporosis and fractures (FDA Femara label); chronic use warrants DEXA monitoring.
- Hot flushes and increased sweating: a vasomotor symptom of estrogen deficiency and one of the most common clinically documented side effects of letrozole.
- Mood disturbance and fatigue: low estrogen can cause depressed mood, irritability, sleep disturbance and marked tiredness.
- Headache, dizziness and nausea: common, usually mild-to-moderate symptoms during treatment, more so at the higher (2.5 mg/day) gyno-reversal dose.
Contraindications · 7
- Pregnancy and breastfeeding: Pregnancy Category X, blocking estrogen synthesis can cause severe fetal harm and miscarriage, so it is absolutely contraindicated in pregnant women.
- Premenopausal women (endocrine status): due to reflex gonadotropin rise and ovarian dysfunction, it is not indicated for premenopausal female use without proper endocrine supervision.
- Severe hepatic or renal impairment: because of hepatic CYP metabolism and renal excretion, clearance is reduced in severe liver or kidney disease, so use is contraindicated or requires close supervision.
- Pre-existing osteoporosis or low bone density: letrozole further lowers BMD, so it should be avoided with established osteoporosis or fracture risk, or used only with bone protection and DEXA monitoring.
- Already low or crashed estradiol: if E2 is already low or symptomatic hypoestrogenism is present, starting or continuing letrozole is inadvisable – do not start without a baseline E2 (sensitive LC-MS/MS) measurement, especially in AI-naive users.
- Concurrent tamoxifen (Nolvadex): due to a pharmacokinetic interaction tamoxifen lowers letrozole plasma levels, so co-administration is not recommended (and in AAS-PCT it also increases the risk of crashing E2).
- Marked untreated adverse lipid profile or cardiovascular disease: because of the HDL-lowering effect it warrants caution or contraindication with existing dyslipidaemia or heart disease without lipid-panel monitoring.
Related Performance Compounds
Same therapeutic category
Studies
Related research and clinical findings
A comparison of letrozole and tamoxifen in postmenopausal women with early breast cancer.
Breast International Group (BIG) 1-98 Collaborative Group, Thürlimann B, Keshaviah A, Coates AS, Mouridsen H, Mauriac L, Forbes JF, Paridaens R, Castiglione-Gertsch M, Gelber RD, Rabaglio M, Smith I, Wardley A, Price KN, Goldhirsch A
The discovery and mechanism of action of letrozole.
Bhatnagar AS
Longer-term outcomes of letrozole versus placebo after 5 years of tamoxifen in the NCIC CTG MA.17 trial: analyses adjusting for treatment crossover.
Jin H, Tu D, Zhao N, Shepherd LE, Goss PE
Influence of letrozole and anastrozole on total body aromatization and plasma estrogen levels in postmenopausal breast cancer patients evaluated in a randomized, cross-over study.
Geisler J, Haynes B, Anker G, Dowsett M, Lønning PE
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The information here is for educational and scientific purposes only. Performance-enhancing compounds (AAS, prohormones, stimulants, doping agents) are illegal without prescription in Hungary and most of the EU, and carry serious health and legal risks. WADA bans them in competitive sport. This is NOT a usage guide, and we do not encourage any illegal use. If you do use them, medical supervision and regular bloodwork are ESSENTIAL. Severe endocrine, cardiovascular, hepatic and psychiatric side effects are possible.