U.S. flag

An official website of the United States government

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Drugs and Lactation Database (LactMed®) [Internet]. Bethesda (MD): National Institute of Child Health and Human Development; 2006-.

Cover of Drugs and Lactation Database (LactMed®)

Drugs and Lactation Database (LactMed®) [Internet].

Show details


Last Revision: January 18, 2021.

Estimated reading time: 1 minute

CASRN: 61-32-5

image 134970879 in the ncbi pubchem database

Drug Levels and Effects

Summary of Use during Lactation

Amounts of methicillin ingested by the infant in breastmilk are small and would not be expected to cause any adverse effects. Occasionally disruption of the infant's gastrointestinal flora, resulting in diarrhea or thrush have been reported with penicillins, but these effects have not been adequately evaluated. Methicillin is acceptable in nursing mothers.

Drug Levels

Maternal Levels. Intramuscular injections of methicillin 1 gram every 6 hours for 6 to 9 days in 17 women with monolateral mastitis yielded methicillin levels in milk that reached a peak of 0.31 mg/L at 3 hours after the first dose on the first day in the healthy breast, and much lower levels in the breast with mastitis. After the last dose, peak milk levels of 1.53 mg/L occurred 6 hours after the dose. Methicillin milk levels from both breasts were equal.[1]

After a single intramuscular dose of 1 gram of methicillin in 3 women, milk levels ranged from 0.2 to 0.3 mg/L between 1 and 6 hours.[2]

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

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

Alternate Drugs to Consider

(Methicillin-resistant Staph. aureus) Doxycycline, Linezolid, Minocycline, Trimethoprim-Sulfamethoxazole, Vancomycin


Kulakov VI, Zak IR, Kulikova NN, et al. Antibiotiki. 1981;26:110–3. [Body pharmacokinetics of methicillin, oxacillin and cephaloridine in puerperal mastitis] [PubMed: 7212690]
Matsuda S. Transfer of antibiotics into maternal milk. Biol Res Pregnancy Perinatol. 1984;5:57–60. [PubMed: 6743732]

Substance Identification

Substance Name


CAS Registry Number


Drug Class

Breast Feeding


Anti-Infective Agents

Antibacterial 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.

Copyright Notice

Attribution Statement: LactMed is a registered trademark of the U.S. Department of Health and Human Services.

Bookshelf ID: NBK501023PMID: 30000082


Related information

Similar articles in PubMed

  • Review Cloxacillin.[Drugs and Lactation Database (...]
    Review Cloxacillin.
    . Drugs and Lactation Database (LactMed®). 2006
  • Review Dicloxacillin.[Drugs and Lactation Database (...]
    Review Dicloxacillin.
    . Drugs and Lactation Database (LactMed®). 2006
  • Review Penicillin G.[Drugs and Lactation Database (...]
    Review Penicillin G.
    . Drugs and Lactation Database (LactMed®). 2006
  • Review Penicillin V.[Drugs and Lactation Database (...]
    Review Penicillin V.
    . Drugs and Lactation Database (LactMed®). 2006
  • Review Ampicillin.[Drugs and Lactation Database (...]
    Review Ampicillin.
    . Drugs and Lactation Database (LactMed®). 2006
See reviews...See all...

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...