Letrozole is taken as a daily pill for five days early in your menstrual cycle, typically starting on cycle day 3. The standard starting dose is 2.5 mg per day, though your doctor may prescribe 5 mg or 7.5 mg depending on your situation. It’s currently the recommended first-line fertility medication for women with polycystic ovary syndrome (PCOS), and it’s also widely used for unexplained infertility.
How Letrozole Helps You Ovulate
Letrozole works by temporarily blocking the enzyme that converts hormones into estrogen. When your estrogen levels drop, your brain interprets this as a signal to produce more follicle-stimulating hormone (FSH). That surge of FSH is what stimulates your ovaries to develop and release an egg.
Unlike some older fertility medications that directly block estrogen receptors throughout your body, letrozole simply reduces estrogen production for a short window. This distinction matters because it means letrozole doesn’t interfere with estrogen’s effects on your uterine lining. In fact, women who switched from clomiphene citrate to letrozole in one study saw their endometrial thickness increase from an average of 5.7 mm to nearly 9 mm, along with improved blood flow to the uterus. A thicker, well-nourished uterine lining creates better conditions for a fertilized egg to implant.
Exactly When and How to Take It
Day 1 of your cycle is the first day of your period (full flow, not spotting). You’ll start letrozole on cycle day 3 and take one pill each day through cycle day 7, for a total of five days. Take it at roughly the same time each day. It doesn’t need to be taken with food, and morning or evening both work fine.
Most clinics start you at 2.5 mg daily. If you don’t ovulate at that dose, your doctor will likely increase it to 5 mg for the next cycle, and potentially up to 7.5 mg after that. A large analysis of over 4,200 cycles found that all three dose levels were used regularly, with dose escalation based on how your ovaries respond.
What Happens After the Five Days
Once you finish the medication, your body does the rest. Your ovaries continue developing the follicle(s) that letrozole stimulated, and ovulation typically occurs sometime between cycle day 14 and day 20, though this varies.
Starting around day 10 or 11, you’ll want to begin using ovulation predictor kits (urine test strips) every morning to detect your LH surge, the hormonal spike that signals ovulation is about 24 to 36 hours away. This is your window for timed intercourse or an insemination procedure.
Your doctor may also schedule a monitoring ultrasound around cycle day 12 or 13 to check follicle size and count. This helps confirm that the medication is working and that you’re not developing too many mature follicles, which would increase the risk of multiples. Some clinics monitor every cycle, while others only monitor the first cycle at a new dose to establish your response pattern.
Success Rates and What to Expect
Letrozole doesn’t guarantee pregnancy on the first cycle. In a major NIH-funded trial of 750 women with PCOS, the cumulative ovulation rate over up to five cycles was 62% with letrozole, compared to 48% with clomiphene citrate. The cumulative live birth rate was 28% for letrozole versus 19% for clomiphene. These numbers reflect multiple attempts, so any single cycle has a lower probability. Most doctors will try letrozole for three to six cycles before changing course.
One significant advantage of letrozole is a lower risk of twins or triplets. Across more than 7,000 treatment cycles in one large study, the multiple pregnancy rate was 4.8% with letrozole compared to 9.4% with clomiphene. No triplet pregnancies occurred in the letrozole group, while six were recorded with clomiphene. This is partly because letrozole tends to produce one or two mature follicles rather than several.
Common Side Effects
Because you’re only taking letrozole for five days at a low dose, side effects are generally mild and short-lived. The most frequently reported ones include:
- Hot flushes and sweating, caused by the temporary drop in estrogen
- Fatigue
- Nausea or reduced appetite
- Mild muscle or bone aches
- Headaches
- Low mood
These side effects typically resolve within a few days of finishing the medication. Letrozole has a half-life of about 45 hours, meaning it clears from your system well before implantation would occur. Early concerns about birth defects, raised at a conference in 2005, were not supported by subsequent research. A large study published in Human Reproduction found no increased risk of major congenital anomalies or adverse pregnancy outcomes following letrozole use.
If Letrozole Doesn’t Work at First
Not responding to the initial dose is common and doesn’t mean the medication has failed entirely. The first adjustment is usually increasing the dose. If you started at 2.5 mg and didn’t ovulate, moving to 5 mg or 7.5 mg for the next cycle is standard.
If you still don’t ovulate at the highest dose, your doctor has several options. One approach that has shown promise is extending the course from five days to seven or even ten days at 5 mg. A study of 69 women with PCOS who didn’t ovulate on the standard five-day protocol found that extending to seven days, and then ten days if needed, improved follicle development and pregnancy rates. Another option is combining letrozole with a second oral medication to amplify the effect.
Beyond oral medications, the next steps typically include injectable hormone treatments or IVF. Some providers also consider a minor surgical procedure called ovarian drilling for women with PCOS who remain resistant to oral medications. Your doctor will help you decide when it makes sense to move on from letrozole based on how many cycles you’ve attempted and your individual circumstances.
Why Letrozole Over Clomiphene
For decades, clomiphene citrate was the default first-line fertility drug. That changed in 2018 when international guidelines from ESHRE, ASRM, and the Australian PCOS research center jointly recommended letrozole as the preferred first-line treatment for PCOS-related infertility. The reasons are straightforward: higher ovulation and live birth rates, a friendlier effect on the uterine lining, and a lower chance of multiples.
Clomiphene blocks estrogen receptors, which can thin the endometrium and dry up cervical mucus in some women. Letrozole avoids both of these problems because it works upstream, reducing estrogen production rather than blocking it at the tissue level. For women without PCOS, the evidence is less clear-cut, but many fertility specialists now use letrozole as a first option regardless of diagnosis.