Letrozole helps you get pregnant by temporarily lowering estrogen levels, which tricks your brain into producing more of the hormones needed to grow and release an egg. It is now the recommended first-line medication for ovulation induction in women with polycystic ovary syndrome (PCOS), and it’s also used in some cases of unexplained infertility. Here’s how it works, what to expect during treatment, and how effective it actually is.
How Letrozole Triggers Ovulation
Letrozole belongs to a class of drugs called aromatase inhibitors. Aromatase is the enzyme your body uses to make estrogen, and letrozole blocks it. When estrogen drops, your brain registers the dip and responds by releasing more follicle-stimulating hormone (FSH) and luteinizing hormone (LH). That surge of FSH stimulates your ovaries to develop follicles, the fluid-filled sacs that each contain an egg. Once a follicle matures, the LH surge triggers ovulation.
There’s also evidence that letrozole works directly at the ovary. By temporarily increasing local androgen levels around the follicles, it may make them more sensitive to FSH. The net effect is that women who don’t ovulate on their own, or who ovulate irregularly, can produce a mature egg on a predictable timeline.
One important distinction from clomiphene (Clomid), the older ovulation drug: clomiphene blocks estrogen receptors throughout the body, including in the uterus. That anti-estrogen effect can thin the uterine lining, making it harder for an embryo to implant. Letrozole lowers estrogen production without blocking its receptors, so the uterine lining typically stays thicker and more receptive. Multiple studies have confirmed that letrozole produces a thicker endometrium than clomiphene, which is one reason it has largely replaced Clomid as the go-to option for PCOS.
What a Typical Treatment Cycle Looks Like
You’ll take letrozole as a pill for five consecutive days, usually starting on day 3 of your menstrual cycle. The standard starting dose is 2.5 mg per day, though your doctor may increase it to 5 mg or 7.5 mg depending on how your ovaries respond. The medication clears your system quickly, which is why its estrogen-lowering effect is temporary and limited to the window your follicles need to start growing.
Most clinics schedule at least two ultrasounds during a letrozole cycle: one at baseline (before you start the medication) and one a few days after you finish the five-day course. The follow-up ultrasound checks how many follicles are developing and how large they are. If a dominant follicle reaches the right size, your doctor may recommend timed intercourse or, in some cases, an intrauterine insemination (IUI). Some cycles also include a blood test to confirm ovulation occurred.
Effectiveness for PCOS
Letrozole’s strongest evidence is in women with PCOS who aren’t ovulating regularly. The 2023 international evidence-based guidelines, endorsed by the American Society for Reproductive Medicine, state that letrozole “should be the first-line pharmacological treatment for ovulation induction in infertile anovulatory women with PCOS, with no other infertility factors.”
The numbers back that recommendation. In a double-blind randomized trial comparing letrozole to clomiphene in women with PCOS, about 84% of women on letrozole ovulated, compared to 80% on clomiphene. The real difference showed up in pregnancy rates: 61.2% of women on letrozole achieved a clinical pregnancy versus 43% on clomiphene. Among women who did ovulate, 70% of those on letrozole became pregnant compared to about 51% on clomiphene. Live birth rates trended higher with letrozole (48.8% versus 35.4%), though that gap didn’t quite reach statistical significance in this particular trial.
Body weight matters. Women with a BMI under 30 saw the biggest benefit from letrozole, with a clinical pregnancy rate of 68.5% compared to 47.2% on clomiphene. For women with a BMI between 30 and 35, the advantage narrowed and was no longer statistically significant.
Effectiveness for Unexplained Infertility
If you ovulate normally but still haven’t conceived, letrozole’s track record is less impressive. A large study published in the New England Journal of Medicine found that live birth rates were 18.7% with letrozole compared to 32.3% with injectable gonadotropins in women with unexplained infertility. Clomiphene also outperformed letrozole in this population. The idea behind using letrozole for unexplained infertility is to push the ovaries to produce more than one egg per cycle, slightly increasing the odds. But the data suggest that for this specific group, other approaches tend to work better.
Your fertility specialist will weigh the cause of your infertility, your age, and how long you’ve been trying when deciding whether letrozole is the right starting point or whether a different approach makes more sense.
Side Effects During Short-Term Use
Letrozole was originally developed for breast cancer treatment, where women take it daily for years. For fertility, you’re only taking it for five days per cycle, so side effects are generally milder and shorter-lived. Still, the temporary estrogen drop can cause symptoms that feel like a compressed version of menopause.
The most commonly reported side effects (occurring in more than 1 in 100 people) include:
- Hot flashes and sweating
- Fatigue
- Headaches
- Nausea
- Mild muscle or bone aches
- Low mood or irritability
- Difficulty sleeping
Serious side effects are rare, occurring in fewer than 1 in 100 people. Most women find the five-day course tolerable, and symptoms typically resolve within a few days of finishing the medication.
Risk of Twins and Multiples
Any ovulation-stimulating drug carries some risk of multiple pregnancy because more than one follicle may mature. With letrozole, the risk is relatively low. In a large trial, 3.4% of letrozole pregnancies were twins, compared to 7.4% with clomiphene. Triplets or higher-order multiples are extremely rare with letrozole. This lower multiple pregnancy rate is considered one of letrozole’s advantages, since twin and triplet pregnancies carry higher risks for both mother and babies.
The ultrasound monitoring during your cycle helps manage this risk. If too many follicles develop, your doctor may recommend canceling the cycle or converting to timed intercourse rather than IUI to reduce the chance of multiples.
Why Letrozole Replaced Clomid as First Choice
For decades, clomiphene was the default fertility pill. Letrozole’s rise to first-line status came down to three practical advantages. First, it produces higher pregnancy rates in women with PCOS. Second, it doesn’t thin the uterine lining, avoiding a problem that causes some women on clomiphene to ovulate successfully but still fail to conceive because the embryo can’t implant. Third, it carries a lower risk of twins.
Clomiphene is still used, particularly when letrozole doesn’t produce a response or when cost and availability are factors. But for most women with PCOS starting fertility treatment for the first time, letrozole is now where treatment begins.