Letrozole is a breast cancer medicine that’s now a first choice to help women ovulate. Here’s how it works, its side effects, how it compares with Clomid, and what to expect.
Letrozole (brand name Femara) is a pill first approved to treat breast cancer, and now widely used to help women ovulate. For fertility, it’s taken for five days early in your cycle. It’s used “off-label,” meaning the manufacturer never applied for fertility approval, but international guidelines now recommend it as the first choice for women with PCOS who don’t ovulate regularly. The most common letrozole side effects are tiredness, dizziness, headaches and hot flashes, usually mild and brief. It carries a lower chance of twins than Clomid. You’ll take a pregnancy test before each cycle, since it must never be taken during pregnancy.
What is letrozole, and how does it help fertility?
- What it is: an aromatase inhibitor, a medicine that briefly lowers estrogen.
- Why that helps: when estrogen drops, the brain releases more of the hormone that tells the ovaries to grow and release an egg.
- Its first use: treating breast cancer, where it’s taken long-term. For fertility, the course is just five days.
- Who it’s for: most often women with PCOS or irregular ovulation, and sometimes women with unexplained infertility, often together with insemination (IUI).
- How it’s taken: one tablet a day for five days, starting a few days after your period begins. Your doctor sets the dose and may adjust it each cycle.
How well does it work?
In a major trial of 750 women with PCOS, women taking letrozole for up to five cycles had more live births than those taking Clomid (about 27% versus 19%) and ovulated more often. Most pregnancies with letrozole happen within the first three or four cycles. If you have PCOS, our guide to metformin for PCOS covers another common treatment, and for general timing, see how long does it take to get pregnant? To check whether a cycle worked, see how to know if you’re ovulating.
Letrozole side effects
Most women have mild side effects, or none. The most common are:
- tiredness
- dizziness
- headaches
- hot flashes
- breast tenderness
- bloating or mild pelvic discomfort around ovulation
They usually appear during the five days of tablets and fade soon after.
Foods to avoid while taking letrozole: there are no specific foods to avoid. Follow the usual advice when trying to conceive: a balanced diet, folic acid, little or no alcohol, and moderate caffeine.
Letrozole or Clomid?
| Letrozole | Clomid | |
|---|---|---|
| Original use | Breast cancer | Ovulation induction |
| FDA-approved for fertility | No (off-label, but guideline-recommended for PCOS) | Yes |
| Live births in PCOS trial | About 27% | About 19% |
| Twins | About 4 in 100 | About 7 in 100 |
| Common side effects | Tiredness, dizziness | Hot flashes, mood changes |
| Effect on uterine lining | Doesn’t thin it | Can thin it |
Safety
- A negative pregnancy test is needed before each cycle. Letrozole must not be taken during pregnancy. Its short course means it has left your body by the time you could conceive.
- Ovulation is usually monitored with blood tests or ultrasound, especially in the first cycle.
- Early concerns about birth defects came from one small report. Larger studies, including the major trial, found no significant difference compared with Clomid.
When to call your doctor
- severe pelvic pain or swelling, rapid weight gain or breathlessness: rare signs of the ovaries overreacting
- vision changes, such as spots or flashes
- heavy bleeding
- a positive pregnancy test, to plan early care
The Chinese medicine view
Chinese medicine sees fertility as resting on Kidney essence and a smooth, regular cycle, and traditionally prepares the body over several months before conception. These are traditional descriptions, not a diagnosis.
Another option while you prepare
Not every woman wants to add more medicines while she’s trying. Fertility Nourish is a sealed herbal pouch worn over the navel, with nothing to swallow, offered as a preconception wellness routine alongside your fertility care, not instead of it. Discuss it with your fertility team first if you’re taking letrozole or other treatment. Stop using the pouch once pregnancy is possible or confirmed. Your first course is covered by our 90-Day Satisfaction Guarantee.
See Fertility Nourish’s directions →
Reviewed October 2026. Author: Qingcai Wang, MD, PhD, developer of the Five Essences external herbal-pouch formulations, with a commercial interest in their sale. General education only; continue your medical care.
Sources: Legro et al., letrozole versus clomiphene for infertility in PCOS, New England Journal of Medicine (2014); 2023 International PCOS (PMOS) Guideline.