Last update Jan. 15, 2020


Very High Risk

Very unsafe. Contraindicated. Use of an alternative or cessation of breastfeeding. Read the Comment.

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.
Oral administration once a day for prolonged periods.

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.

Its long elimination half-life and its administration every 24 hours over many months make it impossible to continue breastfeeding.


We do not have alternatives for Letrozole.

Suggestions made at e-lactancia are done by APILAM team of health professionals, and are based on updated scientific publications. It is not intended to replace the relationship you have with your doctor but to compound it. The pharmaceutical industry contraindicates breastfeeding, mistakenly and without scientific reasons, in most of the drug data sheets.

Jose Maria Paricio, Founder & President of APILAM/e-Lactancia

Your contribution is essential for this service to continue to exist. We need the generosity of people like you who believe in the benefits of breastfeeding.

Thank you for helping to protect and promote breastfeeding.

José María Paricio, founder of e-lactancia.

Other names

Letrozole in other languages or writings:


Letrozole belongs to this group or family:


Main tradenames from several countries containing Letrozole in its composition:


Variable Value Unit
Oral Bioavail. 99 %
Molecular weight 285 daltons
Protein Binding 60 %
VD 1.87 ± 0.47 l/Kg
Tmax 1 hours
48 hours


  1. Hale TW, Rowe HE. Medications & Mothers' Milk. A Manual of Lactation Pharmacology. Springer Publishing Company. 2017
  2. AEMPS. Letrozol. Ficha técnica. 2017 Full text (in our servers)
  3. Novartis. Letrozole. Drug Summary. 2017 Full text (in our servers)
  4. Anderson PO. Cancer Chemotherapy. Breastfeed Med. 2016 May;11:164-5. Abstract Full text (link to original source) Full text (in our servers)

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