Drug Levels and Effects:

Summary of Use during Lactation:

Methicillin is acceptable to use during breastfeeding. Limited information indicates that maternal doses of methicillin up to 4 grams daily produce low levels in milk that are not expected to cause adverse effects in breastfed infants. Occasionally, disruption of the infant's gastrointestinal flora, resulting in diarrhea or thrush, has been reported with penicillins, but these effects have not been adequately evaluated.

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 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.

Possible Effects on Lactation:

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


1. Kulakov VI, Zak IR, Kulikova NN, Smekuna FA. [Body pharmacokinetics of methicillin, oxacillin and cephaloridine in puerperal mastitis]. Antibiotiki. 1981;26:110-3. PMID:7212690
2. Matsuda S. Transfer of antibiotics into maternal milk. Biol Res Pregnancy. 1984;5:57-60. PMID:6743732

Substance Identification:

Substance Name:


CAS Registry Number:


Drug Class:

  • Antiinfective Agents

  • Antibacterial Agents

  • Penicillins

  • Administrative Information:

    LactMed Record Number:


    Last Revision Date:

    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.