Clomid vs Letrozole: Which Works Better for Ovulation?

Clomid (clomiphene citrate) and letrozole are the two oral drugs most often used to trigger ovulation. They reach the same goal by opposite routes, and that difference explains most of what the trials found.

Clomid is a selective estrogen receptor modulator. According to its FDA label, it competes with estrogen for receptor sites, which makes the brain read estrogen as low and release more FSH and LH, ending in a surge that triggers ovulation. Letrozole is an aromatase inhibitor. The Femara label describes it as a "nonsteroidal competitive inhibitor of the aromatase enzyme system" that blocks the conversion of androgens to estrogens. Estrogen actually falls, and the brain responds with more FSH.

In the clomid vs letrozole comparison, the two also clear the body at very different speeds. The aromatase inhibitor has a terminal half-life of about 2 days, according to the Femara label. Clomid contains zuclomiphene, one of its two isomers, which remained detectable for more than a month after a single dose in label studies.

There is a regulatory difference too. Clomid is FDA approved for ovulatory dysfunction in women who want to get pregnant. Letrozole (brand name Femara) is approved only for breast cancer, so its use for fertility in the US is off label. Its label also states that it can cause fetal harm and is contraindicated in pregnancy. The clomid vs letrozole choice therefore starts from two very different drugs. For more on the first, see how Clomid works.

The most important clomid vs letrozole study is PPCOS II, published by Legro and colleagues in the New England Journal of Medicine in 2014. It was a double-blind trial run by the NICHD Reproductive Medicine Network, in which 750 women with polycystic ovary syndrome were randomly assigned to letrozole or clomiphene for up to 5 treatment cycles.

The aromatase inhibitor produced more ovulation and about 44% more live births (rate ratio 1.44). Twins were less than half as common on it, though the difference did not reach statistical significance. There were 4 major congenital anomalies in the letrozole group and 1 with clomiphene, a difference that was not significant (P = 0.65).

Doses followed a fixed protocol described in the trial design paper (Contemporary Clinical Trials, 2012). Clomiphene started at 50 mg a day and letrozole at 2.5 mg a day, both on cycle days 3 to 7, with increases up to 150 mg and 7.5 mg if a woman did not ovulate. Anyone reading letrozole vs clomid results for PCOS should know this is the strongest single piece of evidence, and it favors letrozole.

The picture is different for women who ovulate normally but cannot conceive, a group called unexplained infertility. The AMIGOS trial, published by Diamond and colleagues in the New England Journal of Medicine in 2015, randomly assigned 900 couples to gonadotropin injections, clomiphene or letrozole, each combined with intrauterine insemination, for up to 4 cycles.

In this group, clomiphene gave more live births than letrozole (23.3% vs 18.7%), although the direct difference between the two oral drugs was not statistically significant (P = 0.10). Gonadotropins had the highest birth rate but also a very high rate of multiples: 24 twin and 10 triplet pregnancies. All multiples on clomiphene and letrozole were twins.

The lesson is that the clomid vs letrozole answer depends on the diagnosis. The aromatase inhibitor wins in PCOS, where the problem is a failure to ovulate. In women who already ovulate, the advantage disappears and clomiphene did at least as well. Our Clomid overview explains how the drug is used in each situation.

Professional guidelines on clomid vs letrozole follow the trial results closely. The American Society for Reproductive Medicine endorsed the 2023 International Evidence-based Guideline for PCOS. It recommends that "letrozole should be the first-line pharmacological treatment for ovulation induction in infertile anovulatory women with PCOS" and that it "should be used rather than clomiphene citrate" when there are no other infertility factors. The same guideline notes that letrozole "is still off-label in many countries".

For unexplained infertility, the ASRM 2020 guideline gives clomiphene with intrauterine insemination a strong (Grade A) recommendation. It lists letrozole with insemination as an alternative, with no significant difference in pregnancy or multiple gestation compared with clomiphene. It suggests 3 or 4 cycles of oral drugs with insemination as the best first step for most couples.

Taken together, the guidance points to letrozole first for PCOS and keeps Clomid as a first option for unexplained infertility or when letrozole does not work. Clomid remains the only one of the two with an FDA label for ovulation induction.

The side effect profiles differ in a way that matches the mechanisms. In PPCOS II, hot flushes were more common with clomiphene, while fatigue and dizziness were more common in the aromatase inhibitor group. The trial abstract does not give percentages for these.

The Clomid label gives rates from 8,029 women in clinical studies: ovarian enlargement 13.6%, vasomotor flushes 10.4%, abdominal or pelvic discomfort and bloating 5.5%, nausea and vomiting 2.2%, breast discomfort 2.1%, visual symptoms 1.5% and headache 1.3%. The label warns that visual symptoms may be irreversible and grow more likely with higher doses and longer use. Full details are on our Clomid side effects page.

The endometrium, the lining of the uterus, is another difference. Because Clomid blocks estrogen receptors throughout the body, it can thin the lining. A 2018 meta-analysis by Gadalla and colleagues in Ultrasound in Obstetrics and Gynecology, covering 1,957 women and 3,892 cycles, found the lining was on average 1.39 mm thinner with clomiphene than with letrozole. Pregnancy and live birth were also lower with clomiphene in that analysis, although the authors rated the quality of evidence as very low.

Multiple pregnancy is a common worry with Clomid. The label reports multiple pregnancies in 7.98% of pregnancies in its clinical studies: twins 6.9%, triplets 0.5%, quadruplets 0.3% and quintuplets 0.1%. In PPCOS II, twins occurred in 7.4% of clomiphene pregnancies against 3.4% with letrozole. The same label notes a monozygotic to dizygotic twin ratio of about 1 to 5.

The Clomid label states that increasing the dose or duration beyond 100 mg a day for 5 days is not recommended, and that cyclic therapy should not continue beyond a total of about six cycles. Higher doses used in trials are given under specialist monitoring. Prices for both generic drugs are compared in our Clomid cost guide.

Clomid.org is an independent educational resource. We are not affiliated with any pharmaceutical manufacturer, compounding pharmacy, telehealth company or healthcare provider. This content is for informational purposes only and does not constitute medical advice. Images on this site are illustrative artwork made for editorial purposes; they do not show real products, packaging or labels and are not an advertisement or an offer to sell.