Drug Levels and Effects:

Summary of Use during Lactation:

Oxacillin is acceptable to use during breastfeeding. Limited information indicates that maternal doses of oxacillin 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.

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.

Possible Effects on Lactation:

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


1. Prigot A, Froix CJ, Rubin E. Absorption, diffusion, and excretion of a new penicillin, oxacillin. Antimicrob Agents Annu. 1962;2:402-10.
2. Borderon E, Soutoul JH et al. [Excretion of antibiotics in human milk]. Med Mal Infect. 1975;5:373-6.
3. Peiker G, Schroder S. [The concentration of oxacillin and ampicillin (Penstabil) in mother's milk in puerperal mastitis.] Pharmazie. 1986;41:793-5. PMID:3562514
4. 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
5. Matsuda S. Transfer of antibiotics into maternal milk. Biol Res Pregnancy. 1984;5:57-60. PMID:6743732
6. 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. PMID:9596854

Substance Identification:

Substance Name:


CAS Registry Number:


Drug Class:

  • Antiinfective Agents

  • Antibacterial Agents

  • Penicillins

  • Administrative Information:

    LactMed Record Number:


    Last Revision Date:

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