Letrozole for Ovulation Induction (Fertility) — Reproduction
Aromatase inhibitor used off-label as first-line ovulation induction agent in PCOS. Grade A evidence from NICHD trial.
Overview
Letrozole has emerged as the first-line treatment for ovulation induction in anovulatory infertility, particularly in women with polycystic ovary syndrome (PCOS), despite being FDA-approved only for breast cancer. The landmark NICHD Reproductive Medicine Network trial (Legro et al., NEJM 2014) — a multicenter, double-blind RCT of 750 women with PCOS — demonstrated that letrozole 2.5-7.5 mg daily for 5 days (cycle days 3-7) produced significantly higher ovulation rates (61.7% vs 48.3%), pregnancy rates (27.5% vs 19.1%), and live birth rates (27.5% vs 19.1%) compared to clomiphene citrate, the previous standard of care. ACOG now recommends letrozole as first-line for ovulation induction in PCOS (Practice Bulletin, 2018). Letrozole works by temporarily suppressing estrogen levels, releasing the hypothalamic-pituitary axis from negative feedback and increasing FSH secretion, which drives follicular development. Unlike clomiphene, letrozole does not have anti-estrogenic effects on the endometrium and cervical mucus, potentially explaining its superior pregnancy and live birth rates. Letrozole also produces more monofollicular (single egg) cycles compared to clomiphene, reducing the risk of multiple pregnancies. The drug clears rapidly (half-life ~2 days), so by the time of implantation, drug levels are negligible. Despite initial concerns about teratogenicity (based on a flawed 2005 abstract), subsequent large studies and meta-analyses have confirmed no increased risk of birth defects with letrozole compared to clomiphene or spontaneous conception.
Indications
- Off-label (first-line per ACOG): Ovulation induction in PCOS
- Off-label: Ovulation induction in unexplained infertility
- Off-label: Controlled ovarian stimulation for IUI
- FDA-approved: Breast cancer only (not fertility)
Mechanism of Action
Letrozole temporarily reduces estradiol levels by blocking aromatase, releasing the hypothalamus and pituitary from estrogen-mediated negative feedback
Dosing
| Compound | Dose | Frequency | Notes |
|---|---|---|---|
| Letrozole | 2.5 mg | Daily for 5 days (cycle days 3-7) | Standard starting dose for ovulation induction |
| Letrozole | 5 mg | Daily for 5 days (cycle days 3-7) | Dose escalation if no response to 2.5 mg |
| Letrozole | 7.5 mg | Daily for 5 days (cycle days 3-7) | Maximum dose for ovulation induction |
Evidence Grade
GRADE A
Safety & Contraindications
- NOT FDA-approved for fertility — Category X for breast cancer indication
- No increased birth defect risk demonstrated in large studies (despite initial concerns)
- Hot flashes, headache, fatigue common during treatment
- Ovarian hyperstimulation possible (rare with letrozole — less than with gonadotropins)
- Multiple pregnancy rate lower than clomiphene but not zero
- Short half-life (~2 days) — minimal drug exposure at time of implantation