Letrozole is not recommended while breastfeeding. The drug’s manufacturer advises stopping breastfeeding during treatment and waiting at least 3 weeks after the last dose before nursing again. This guidance is based on limited data showing the drug may pass into breast milk in concerning amounts, along with harm observed in animal studies.
Why Letrozole and Breastfeeding Don’t Mix
Letrozole works by blocking an enzyme that produces estrogen. This sharp drop in estrogen is what makes it effective for treating hormone-receptor-positive breast cancer and for stimulating ovulation in fertility treatment. But estrogen plays a role in maintaining milk production, and suppressing it can interfere with your supply. Beyond the effect on milk itself, the concern is what the drug could do to a nursing infant.
The LactMed database, maintained by the National Institutes of Health, notes that “a minimal amount of information on letrozole excretion into milk indicates that the amounts might be of concern.” There simply haven’t been enough human studies to confirm how much of the drug transfers into breast milk or what effects that exposure could have on a developing baby. The safety concerns come primarily from animal research, where letrozole caused harm in offspring.
How Long Letrozole Stays in Your System
Letrozole has a terminal elimination half-life of about 2 days. That means it takes roughly 2 days for your body to clear half of a single dose. After daily use at the standard 2.5 mg dose, the drug reaches steady-state concentration in 2 to 6 weeks, meaning it builds up in your system over time.
This is why the recommended waiting period after stopping is 3 weeks, not just a few days. It takes multiple half-lives for the drug to fully clear, and that timeline is longer when you’ve been taking it consistently. If you’re on letrozole for breast cancer treatment, which typically lasts 5 to 10 years, breastfeeding during that entire course is off the table.
Short-Course Use for Fertility
If you’re taking letrozole as a fertility medication, the situation looks a bit different from long-term cancer treatment. Fertility protocols typically involve taking letrozole for just 5 days early in your menstrual cycle to trigger ovulation. In that scenario, the drug exposure is brief and the concentration in your body is much lower than with ongoing daily use.
That said, the official recommendation still applies: breastfeeding should be paused during treatment and for 3 weeks after the last dose. For someone using letrozole to conceive while still nursing an older child, this means planning a temporary break from breastfeeding. You would need to pump and discard milk during that window to maintain supply if you plan to resume nursing afterward.
What About Your Milk Supply
Even setting aside the question of infant exposure, letrozole can reduce your milk production. By lowering estrogen levels, it disrupts the hormonal balance that supports lactation. In fact, letrozole has been studied specifically as a tool to suppress milk production in women who choose not to breastfeed after delivery. So taking it while trying to maintain a nursing relationship could undermine your supply independently of any safety concerns for the baby.
Options If You Need Treatment While Nursing
What you can do depends entirely on why you’re taking letrozole. If it’s for fertility, the 3-week pause after a short course is manageable for many families, especially if your child is older and eating solid foods alongside breast milk. If it’s for breast cancer, the treatment timeline is long enough that continuing to breastfeed isn’t realistic, and your oncology team will help you plan the transition.
If you’re exploring fertility treatment and want to minimize disruption to breastfeeding, it’s worth discussing alternative ovulation-induction options with your reproductive endocrinologist. Some medications have more established safety data during lactation, and the best choice depends on your specific fertility diagnosis and how important continued breastfeeding is to your family’s plan.