Is Letrozole compatible with breastfeeding? Do we have alternatives for Letrozole?


June 10, 2018 (Very High Risk)

Selective inhibitor of aromatase (estrogen-synthetase), similar to anastrozole, which inhibits the secretion of estrogens.
It is used during the preoperative phase in positive hormone-receptor breast cancer in order to try breast-conserving surgery.
It is used in infertility treatments due to ovulation problems, luteal phase defects including polycystic ovarian syndrome.
Administered orally once a day.

Since the last update date we have not found published data on its excretion in breastmilk.

Its pharmacokinetic data (low molecular weight, low protein binding and prolonged half-life) make transfer to milk likely in quantities that could be significant.

From pharmacokinetics it is known that after 3 elimination half-lives (T½) 87.5% of the drug is eliminated from the body; after 4 T½, 94%, after 5 T½ 96.9%, after 6 T½ 98.4% and after 7 T½ 99%. From 7 T½ plasma drug concentrations in the body are negligible. In general, a period of five half-lives can be considered a safe waiting period before returning to breastfeeding (Anderson 2016).

Expert authors recommend waiting 10 days (5 T½) after the last dose to restart breastfeeding (Hale 2017 pp. 548), fewer than the manufacturer, which advises 21 days (Novartis 2017). In the meantime, express and discard breast milk regularly.


We do not have alternatives for Letrozole.

Very Low Risk

Compatible. Not risky for breastfeeding or infant.

Low Risk

Moderately safe. Mild risk possible. Follow up recommended. Read the Comment.

High Risk

Poorly safe. Evaluate carefully. Use a safer alternative. Read the Comment.

Very High Risk

Not recommended. Cessation of breastfeeding or alternative.


  • Λετροζόλη (Greek)
  • ليتروزول (Arabic)
  • Летрозол (Cyrillic)
  • 来曲唑 (Chinese)
  • レトロゾール (Japanese)
  • C17 H11 N5 (Molecular formula)
  • 4,4′-(1H-1,2,4-Triazol-1-ylmethylene)dibenzonitrile (Chemical name)
  • L02BG04 (ATC Code/s)


  1. AEMPS. Letrozol. Ficha técnica. 2017 Full text (in our servers)
  2. Novartis. Letrozole. Drug Summary. 2017 Full text (in our servers)
  3. Hale TW, Rowe HE. Medications & Mothers' Milk. A Manual of Lactation Pharmacology. Springer Publishing Company. 2017
  4. Anderson PO. Cancer Chemotherapy. Breastfeed Med. 2016Abstract Full text (link to original source) Full text (in our servers)