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Drugs and Lactation Database (LactMed) [Internet]. Bethesda (MD): National Library of Medicine (US); 2006-.

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Drugs and Lactation Database (LactMed) [Internet].

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Last Revision: July 20, 2020.

Estimated reading time: 2 minutes

CASRN: 10540-29-1

image 134990041 in the ncbi pubchem database

Drug Levels and Effects

Summary of Use during Lactation

Tamoxifen and its active metabolites are detectable in milk and accumulate in milk over time. It can also suppress lactation. Tamoifen should be avoided in nursing mothers.

Drug Levels

Maternal Levels. A nursing mother was diagnosed with breast cancer and started on tamoxifen 20 mg daily. Thirty-nine milk samples were collected over 23 days beforeand after tamoxifen initiation, although the timing of samples with respect to doses was not provided in the abstract. On day 1 of therapy, tamoxifen was detected in a concentration of 15.2 mcg/L. Milk concentrations increased steadily to a concentration of 79.5 mcg/L on day 23. Active metabolites 4-hydroxytamoxifen and endoxifen (4-hydroxy-N-desmethyltamoxifen) also increased over time and were quantifiable by day 18 of therapy.[1]

Infant Levels. Relevant published information was not found as of the revision date.

Effects in Breastfed Infants

Relevant published information was not found as of the revision date.

Effects on Lactation and Breastmilk

Tamoxifen was more effective than placebo in suppressing lactation and preventing engorgement and pain in two trials in postpartum mothers. Neither study stated what, if any, physical methods (e.g., breast binding) were used concurrently.[2,3] In one study of 80 women, tamoxifen 10 mg four times daily for 5 days during the first week postpartum was more effective than placebo in suppressing a rise in serum prolactin after use of a mechanical breast pump after 5 days of treatment, but not on day 3. All of the women in the study had breastfed a previous child.[2] The other study of 150 women used 2 regimens starting within 2 hours of delivery: tamoxifen 30 mg twice daily for 2 days, followed by 10 mg twice daily for 2 days; and 10 mg twice daily for 14 days. More women in the tamoxifen groups had not previously breastfed an infant.[3]

In a case report, a woman with a history of breastfeeding 4 children (the last having been weaned 10 months earlier) began lactating after 1 week of a cancer chemotherapy regimen for breast cancer that included tamoxifen 20 mg/day. Milk production continued for several weeks until tamoxifen was discontinued after which it did not return during 12 more weeks of chemotherapy.[4]


Helland T, Codacci-Pisanelli G, Johansson H, et al. Transfer of tamoxifen into human milk during treatment of endocrine sensitive breast cancer. Clin Pharmacol Ther. 2020;107 Suppl 1:S53. Abstract.
Masala A, Delitala G, Lo Dico G, et al. Inhibition of lactation and inhibition of prolactin release after mechanical breast stimulation in puerperal women given tamoxifen or placebo. Br J Obstet Gynaecol. 1978;85:134–7. [PubMed: 626722]
Shaaban MM. Suppression of lactation by an antiestrogen, tamoxifen. Eur J Obstet Gynecol Reprod Biol. 1975;4:167–9. [PubMed: 1053489]
Favis GR, Alavi JB, Glick JH. Lactation from tamoxifen in breast cancer. Ann Intern Med 1979;90:993-4. Letter. PMID: 220899. [PubMed: 220899]

Substance Identification

Substance Name


CAS Registry Number


Drug Class

Breast Feeding


Antineoplastic Agents

Estrogen Antagonists

Disclaimer: Information presented in this database is not meant as a substitute for professional judgment. You should consult your healthcare provider for breastfeeding advice related to your particular situation. The U.S. government does not warrant or assume any liability or responsibility for the accuracy or completeness of the information on this Site.


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