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 Library of Medicine (US); 2006-.

Cover of Drugs and Lactation Database (LactMed)

Drugs and Lactation Database (LactMed) [Internet].

Show details


Last Revision: October 31, 2018.

Estimated reading time: 2 minutes

CASRN: 66-79-5

Chemical structure

Drug Levels and Effects

Summary of Use during Lactation

Limited information indicates that oxacillin produces 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 have been reported with penicillins, but these effects have not been adequately evaluated. Oxacillin is acceptable in nursing mothers.

Drug Levels

Maternal Levels. After 3 to 4 doses of oxacillin 500 mg orally in 10 women, the highest level detected was 0.2 mg/L in two women 4 hours after the dose. In the other 8 women, milk levels were less than 0.2 mg/L at times between 1 and 3.75 hours after the dose.[1]

One woman received oral oxacillin 3 grams daily. After the third dose, milk oxacillin levels ranged from 0.04 to 0.1 mg/L, with the peak occurring at 3 hours after the dose. The authors estimated that a breastfed infant would receive 0.3 mg daily of oxacillin in milk.[2]

In 10 women with mastitis who received 1 gram of oxacillin orally 3 times daily for 5 to 6 days, the average milk concentrations ranged from 0.43 to 0.68 mg/L.[3]

Repeated injections of oxacillin 1 gram intramuscularly every 6 hours for 6 to 9 days in 15 women with monolateral mastitis yielded oxacillin milk levels that reached a peak of 0.49 mg/L at 3 hours after the first dose on the first day in the healthy breast, and somewhat lower levels in the breast with mastitis. After the last dose, peak milk levels of 0.64 mg/L occurred 6 hours after the dose in the unaffected breast with slightly lower levels in the breast with mastitis.[4]

After a single intramuscular dose of 500 mg of oxacillin in 2 women, milk levels ranged from 0.18 to 0.7 mg/L between 1 and 4 hours, with the peak occurring at 2 to 4 hours after the dose. Only a trace was detectable at 6 hours after the dose.[5]

In 15 women given a single 1 gram dose of oxacillin intravenously, milk levels averaged 0.68 mg/L at 2 hours after the dose.[6]

Infant Levels. Ten women with mastitis received 1 gram of oxacillin orally 3 times daily for 5 to 6 days. Oxacillin was detectable in the urine in 5 of 6 of the breastfed infants in whom it was measured. Urine concentrations ranged from 0.2 to 3.7 mg/L.[3]

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


Prigot A, Froix CJ, Rubin E. Absorption, diffusion, and excretion of a new penicillin, oxacillin. Antimicrob Agents Annu. 1962;2:402-10.
Borderon E, Soutoul JH et al. [Excretion of antibiotics in human milk]. Med Mal Infect. 1975;5:373-6.
Peiker G, Schroder S. [The concentration of oxacillin and ampicillin (Penstabil) in mother's milk in puerperal mastitis.] Pharmazie. 1986;41:793-5. [PubMed: 3562514]
Kulakov VI, Zak IR, Kulikova NN, Smekuna FA. [Body pharmacokinetics of methicillin, oxacillin and cephaloridine in puerperal mastitis]. Antibiotiki. 1981;26:110-3. [PubMed: 7212690]
Matsuda S. Transfer of antibiotics into maternal milk. Biol Res Pregnancy. 1984;5:57-60. [PubMed: 6743732]
Zhang Y, Zhang Q, Xu Z. [Tissue and body fluid distribution of antibacterial agents in pregnant and lactating women]. Zhonghua Fu Chan Ke Za Zhi. 1997;32(5):288-92. [PubMed: 9596854]

Substance Identification

Substance Name


CAS Registry Number


Drug Class

  • Breast Feeding
  • Anti-Infective Agents
  • Antibacterial Agents
  • Penicillins

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.


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 Carbenicillin Indanyl Disodium[Drugs and Lactation Database (...]
    Review Carbenicillin Indanyl Disodium
    . Drugs and Lactation Database (LactMed). 2006
  • Review Penicillin G[Drugs and Lactation Database (...]
    Review Penicillin G
    . Drugs and Lactation Database (LactMed). 2006
  • Review Nafcillin[Drugs and Lactation Database (...]
    Review Nafcillin
    . 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...