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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: September 21, 2020.

Estimated reading time: 2 minutes

CASRN: 51481-61-9

image 135002058 in the ncbi pubchem database

Drug Levels and Effects

Summary of Use during Lactation

Maternal cimetidine doses of 1000 to 1200 mg daily result in infant dosages that are much less than reported neonatal dosages of 5 to 10 mg/kg daily. Cimetidine would not be expected to cause any adverse effects in breastfed infants, especially if the infant is older than 2 months. However, because of its potential for causing hepatic enzyme inhibition, other drugs might be preferred.

Drug Levels

Maternal Levels. After a single oral dose of 400 mg of cimetidine, the peak milk level of 5 mg/L occurred about 3 hours after the dose and was about equal to the peak serum level in a woman who had been breastfeeding for 6 months. Milk levels remained higher than serum levels for the duration of the dosing interval. With a multiple-dose regimen of 200 mg 3 times daily and 400 mg at bedtime, milk levels just prior to the doses were relatively constant between 4.9 and 6 mg/L.[1] These levels indicate that a fully breastfed infant would receive between 0.74 and 0.9 mg/kg daily.

Twelve healthy, lactating human volunteers who were 6 to 45 weeks postpartum were given single doses of cimetidine 100 mg, 600 mg and 1200 mg on 3 different days within a 15-day period. Blood and milk samples were donated before each dose and 11 times over the next 24 hours. Average peak cimetidine levels in milk occurred at 3.3 hours after the dose. The dose-normalized peak milk level was 2.5 to 3.1 mg/L for each 100 mg of dosage. The half-life in milk was 2.5 hours, which was longer than in serum (1.4 hours).[2] Using data from the paper, a fully breastfed infant whose mother was receiving 1200 mg daily would receive an average dose of 1.4 mg/kg daily, which is in the neonatal dosage range. Average milk values equated to a steady-state weight-adjusted relative infant dose of 1.1% of the maternal weight-adjusted dose

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

Histamine H2-receptor blockade is known to stimulate prolactin secretion. In addition, cimetidine may have additional, nonspecific actions that stimulate prolactin secretion.[3] Oral cimetidine doses of 400 mg 4 times daily increased serum prolactin by 50 to 112% in 6 patients. Cimetidine caused dose-related gynecomastia and galactorrhea in men and nonnursing women.[4-6] The prolactin level in a mother with established lactation may not affect her ability to breastfeed.

Alternate Drugs to Consider

Antacids, Famotidine, Omeprazole, Pantoprazole, Sucralfate


Somogyi A, Gugler R. Cimetidine excretion into breast milk. Br J Clin Pharmacol 1979;7:627-9. Letter. PMID: 465286. [PMC free article: PMC1429661] [PubMed: 465286]
Oo CY, Kuhn RJ, Desai N, et al. Active transport of cimetidine into human milk. Clin Pharmacol Ther. 1995;58:548–55. [PubMed: 7586949]
Knigge UP. Histaminergic regulation of prolactin secretion. Dan Med Bull. 1990;37:109–24. [PubMed: 2188799]
Delle Fave FG, Tamburrano G, De Magistris L, et al. Gynaecomastia with cimetidine. Lancet 1977;309:1319. PMID: 68422. [PubMed: 68422]
Bateson MC, Browning MCK, Maconnachie A. Galactorrhoea with cimetidine. Lancet 1977;310:247-8. Letter. PMID: 69853. [PubMed: 69853]
García Rodríguez LA, Jick H. Risk of gynaecomastia associated with cimetidine, omeprazole, and other antiulcer drugs. BMJ. 1994;308:503–6. [PMC free article: PMC2542783] [PubMed: 8136667]

Substance Identification

Substance Name


CAS Registry Number


Drug Class

Breast Feeding


Anti-Ulcer Agents

Histamine H2 Antagonists

Gastrointestinal Agents

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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