Letrozole
Letrozole is a highly potent non-steroidal (reversible) aromatase inhibitor. It is extremely effective at lowering oestrogen — so effective that it is the AI most likely to be over-used, crashing oestradiol to symptomatic levels. It has legitimate niche uses (established gynaecomastia reversal, ovulation induction) but for routine on-cycle oestrogen control it is easy to badly over-suppress.
01 Overview
Letrozole inhibits aromatase competitively and can suppress whole-body oestrogen production by more than 98%, more completely than anastrozole at comparable doses. This potency is a double-edged sword: milligram-for-milligram it is very strong, and the typical dosing error is running too much, which crashes oestradiol and produces joint pain, low libido, lethargy, adverse lipid shifts and mood disturbance.
It has evidence-backed uses beyond simple oestrogen management: it can reduce established (not merely early) gynaecomastia in some cases, and it is a first-line ovulation-induction agent. In men its potency means it should be dosed conservatively and titrated to a sensitive oestradiol assay rather than to symptoms.
02 Mechanism
Reversible, competitive non-steroidal inhibitor of the aromatase (CYP19A1) enzyme, blocking androgen-to-oestrogen conversion.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Oestrogen control (low) | 0.25–0.5 mg/day | Oral | Off-label; frequently dosed every other day or twice weekly. Very easy to over-suppress. |
| Gynaecomastia reversal | 1.25–2.5 mg/day | Oral | Short courses only; high risk of crashing oestradiol if continued. |
| Breast cancer (label) | 2.5 mg/day | Oral | FDA-approved dose for post-menopausal breast cancer. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Profound oestradiol suppressionThe most potent commonly used AI; strongly controls oestrogenic side effects. | up to -98% | Clinical | |
| Gynaecomastia reductionCan reduce established glandular tissue in some men, not only prevent new growth. | partial regression | Clinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Oestradiol crashBecause of its potency letrozole is the AI most likely to over-suppress: loss of libido, erectile dysfunction, fatigue, depressed mood and joint pain. | Moderate | Very common when over-dosed | Clinical | |
| ArthralgiaJoint pain and stiffness, a class effect amplified by letrozole's potency. | Moderate | Common | Clinical | |
| Adverse lipid changesReduced HDL and worsened lipid profile with sustained suppression. | Moderate | Common | Clinical |
06 Commonly used with
What letrozole is combined with, and why. This is about intent rather than safety — the interactions table below covers what is dangerous. Nothing here is listed without what it costs.
| Compound | Frequency | Purpose & trade-off |
|---|---|---|
| Support & ancillaries | ||
| Testosteronehigh dose | Reserved for high-dose testosterone where weaker AIs fail, as letrozole is the most potent estrogen suppressor available.Trade-off: Its potency makes it very easy to crash estrogen to zero, wrecking lipids, joints, mood and libido. | |
| Dianabolgyno rescue | Used to reverse early gynecomastia flaring from strong oral aromatisers when milder AIs are not enough.Trade-off: The estrogen crash needed to reverse gyno commonly brings lethargy, joint pain and killed libido. | |
| Nandroloneprogestin control | Sometimes added on nandrolone cycles to lower estrogen, which reduces progesterone-driven gyno risk.Trade-off: It does not touch progesterone itself and the estrogen crash can worsen nandrolone's already poor libido profile. | |
| Tamoxifengyno protocol | Combined short-term with a SERM to attack active gynecomastia from both estrogen production and receptor sides.Trade-off: The aggressive dual suppression produces pronounced low-estrogen side effects and rebound risk on discontinuation. | |