Last update Jan. 15, 2019


Very High Risk

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

Anastrozole is a selective aromatase inhibitor (estrogen synthetase), which inhibits the secretion of estrogen.
Indicated in the treatment of breast cancer in postmenopause.
Oral administration once a day for prolonged periods.

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

Its pharmacokinetic data (low molecular weight, low protein binding and prolonged half-life) enables transfer to breastmilk in amounts which could be significant.

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

According to this data, the interruption would be for a period of between 10.4 days (5 T½) to 14.6 days (7 T½) after the administration of the last dose, which coincides with the estimates of various authors: 10 days (Schaefer 2007, p 741) to 15 days (Hale 2017, p 66).

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 Anastrozole.

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

Anastrozole in other languages or writings:


Anastrozole belongs to this group or family:


Main tradenames from several countries containing Anastrozole in its composition:


Variable Value Unit
Oral Bioavail. 100 %
Molecular weight 293 daltons
Protein Binding 40 %
pKa 2.01 -
Tmax 2 hours
40 - 50 hours


  1. Hale TW, Rowe HE. Medications & Mothers' Milk. A Manual of Lactation Pharmacology. Springer Publishing Company. 2017
  2. Anderson PO. Cancer Chemotherapy. Breastfeed Med. 2016 May;11:164-5. Abstract Full text (link to original source) Full text (in our servers)
  3. AEMPS-Stada. Anastrozol. Ficha técnica. 2016 Full text (in our servers)
  4. Astra-Zeneca. Anastrozole. Data Sheet. 2010 Full text (in our servers)
  5. Schaefer C, Peters P, Miller RK. Drugs During Pregnancy and Lactation. Treatment options and risk assessment. Elsevier, second edition. London. 2007

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