Letrozole beat clomiphene for live births in PCOS, the trial that reset first-line care

For women with polycystic ovary syndrome trying to conceive, letrozole — a breast-cancer drug used off-label to induce ovulation — produced more live births than clomiphene, the long-standing first-line treatment: 27.5% versus 19.1% over up to five cycles in the head-to-head trial that reset practice [s1]. That randomised comparison is why the 2023 international PCOS guideline now names letrozole, not clomiphene, the first-line drug when the goal is pregnancy [s2].

Why the comparison mattered

PCOS is the most common cause of anovulation — cycles in which no egg is released — and for decades clomiphene, an oestrogen-receptor modulator, was the reflex first prescription for women with the condition who wanted to conceive. Letrozole works differently, as an aromatase inhibitor that lowers oestrogen and prompts the pituitary to drive ovulation, and smaller studies had hinted it might do better. The question was worth a definitive trial because the stakes are a pregnancy, and because letrozole's use here is off-label, which had raised — and, in this trial, largely answered — safety questions about birth defects. Our survey of the evidence base for PCOS management sets this drug question in the wider picture.

What the trial did

The double-blind, multicentre trial randomly assigned 750 women in a 1:1 ratio to letrozole or clomiphene for up to five treatment cycles, with visits to confirm ovulation and pregnancy [s1]. Eligibility was specific: PCOS by modified Rotterdam criteria (anovulation with either hyperandrogenism or polycystic ovaries), age 18 to 40, at least one patent fallopian tube and a normal uterine cavity, and a male partner with a sperm concentration of at least 14 million per millilitre, with both partners agreeing to regular intercourse during the study [s1]. The primary outcome was live birth during the treatment period — not ovulation, not pregnancy, but a baby [s1].

What it found

Women who received letrozole had more cumulative live births than those on clomiphene: 103 of 374 (27.5%) versus 72 of 376 (19.1%), P = 0.007, a rate ratio of 1.44 (95% CI 1.10 to 1.87) [s1]. The mechanism showed up upstream, in ovulation: letrozole produced ovulation in 834 of 1,352 treatment cycles (61.7%) against 688 of 1,425 cycles (48.3%) for clomiphene, P < 0.001 [s1].

The numbers that reassure are the ones that stayed level. There were no significant between-group differences in pregnancy loss (49 of 154 pregnancies, 31.8%, with letrozole versus 30 of 103, 29.1%, with clomiphene) or in twin pregnancy (3.4% versus 7.4%) [s1]. On the safety question that had shadowed letrozole, there was no significant difference in overall congenital anomalies, though the honest detail is that there were four major congenital anomalies in the letrozole group against one in the clomiphene group (P = 0.65) — a difference that did not reach significance in a trial not powered to detect rare events, which is a limit to state rather than a reassurance to bank [s1]. Side-effect profiles differed in character: clomiphene brought more hot flushes, letrozole more fatigue and dizziness [s1].

What it does and does not settle

The trial settles the comparison it set out to make — letrozole beats clomiphene on live births and ovulation in this well-defined PCOS population — and it does so with the outcome that matters, a live birth, rather than a surrogate [s1]. That is why it moved guidelines. The 2023 International Evidence-based Guideline for PCOS recommends letrozole as the first-line pharmacological treatment for ovulation induction in women with the condition [s2].

What it does not do is speak to women outside its entry criteria — those with blocked tubes, a male-factor problem, or outside the 18-to-40 band — or to how letrozole compares with newer or combination approaches. The absolute gain is worth keeping in proportion: even in the winning arm, fewer than three in ten women had a live birth over five cycles [s1], a reminder that ovulation induction improves the odds rather than guaranteeing a pregnancy, a framing our look at how fertility changes with age develops further.

What to watch

Longer-term registry data on congenital anomalies with off-label letrozole, given the small, non-significant numeric imbalance in this trial [s1], and whether the guideline's first-line status holds as ovulation-induction options widen [s2]. This article describes trial and guideline evidence and is not medical advice.

Sources

• Letrozole versus Clomiphene for Infertility in the Polycystic Ovary Syndrome — New England Journal of Medicine, 2014-07-09
• Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome — Journal of Clinical Endocrinology & Metabolism, 2023-08-04