What is letrozole?
Letrozole is an aromatase inhibitor, a tablet that lowers estrogen production by blocking the enzyme that makes it. Originally a breast cancer drug, it is widely used off-label to induce ovulation: lower estrogen prompts the brain to release more FSH, which helps a follicle mature. It is often the first choice for ovulation induction in PCOS.
How letrozole works for ovulation
Taken for five days early in the cycle (often days 3 to 7), letrozole briefly lowers estrogen. The hypothalamus responds by boosting FSH, so one or more follicles grow. When the drug stops, estrogen rises again and ovulation usually follows around days 12 to 16. See ovulation induction.
Letrozole vs clomiphene
| Letrozole | [Clomiphene](/glossary/clomiphene) (Clomid) | |
|---|---|---|
| How it works | lowers estrogen production | blocks estrogen receptors in the brain |
| Live birth rate in PCOS | higher in large trials | lower in the same trials |
| Effect on lining and mucus | less thinning | can thin the lining and dry mucus |
| Twins | slightly lower risk | about 5 to 8% |
You can estimate likely ovulation with the Clomid and letrozole ovulation calculator.
Side effects and mood
Common effects are hot flashes, headaches, tiredness, dizziness and breast tenderness, usually mild and short-lived. Some people notice mood swings or low mood during the tablet days or in the two-week wait. Tracking mood across treatment cycles helps you and your doctor see what is the medicine and what is the stress of trying.
Use letrozole only under medical supervision, with a pregnancy test before each new cycle. Contact your doctor for severe pelvic pain or bloating, which can signal ovarian overstimulation.
Frequently asked questions
Is letrozole approved for fertility?
It is used off-label in most countries, but major guidelines, including ESHRE and ACOG, recommend it as a first-line option for ovulation induction in PCOS.
How many cycles of letrozole can I try?
Often up to around six ovulatory cycles before reviewing the plan with a fertility specialist.
Sources
- Legro RS et al. Letrozole versus clomiphene for infertility in the polycystic ovary syndrome. NEJM (2014)
- MedlinePlus: Letrozole
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Clomid ovulation calculator: when will I ovulate on Clomid or letrozole?Free Clomid ovulation calculator: enter cycle day 1 and your first pill day to see when you'll likely ovulate and when to start LH tests. Letrozole too.
What is clomiphene (Clomid)?Clomiphene (Clomid) is a pill that triggers ovulation. How it works, when ovulation happens after taking it, side effects including mood, and the risks.Ovulation induction: definition and meaningOvulation induction uses medicine to help the ovaries release an egg when you don't ovulate regularly. Who it helps, the main drugs, and monitoring.
What is PCOS (polycystic ovary syndrome)?PCOS (polycystic ovary syndrome) is a common hormone condition causing irregular periods, high androgens and ovulation problems. Signs, diagnosis and care.Anovulation: definition and meaningAnovulation means a cycle where no egg is released. What it is, why it happens, how it changes your period and mood, and when to talk to a doctor.