Animal studies have failed to reveal evidence of fetal harm due to penicillin G. No positive evidence of adverse effects on the fetus observed during human use of penicillins; however, there are no controlled data in human pregnancy.

US FDA pregnancy category Not Assigned: The US FDA has amended the pregnancy labeling rule for prescription drug products to require labeling that includes a summary of risk, a discussion of the data supporting that summary, and relevant information to help health care providers make prescribing decisions and counsel women about the use of drugs during pregnancy. Pregnancy categories A, B, C, D, and X are being phased out.

This drug should be used during pregnancy only if clearly needed.

US FDA pregnancy category: Not assigned.

See references

Use is considered acceptable; caution is recommended.

Excreted into human milk: Yes

Comments:
-The effects in the nursing infant are unknown.
-Limited data indicate that low levels in milk from single maternal doses are not expected to cause harmful effects in nursing infants.

Soluble penicillin G is excreted in breast milk.

Single or multiple doses of penicillin G (100,000 units IM once or every 2 hours for 6 hours) were administered to 10 women. At 1 to 2 hours after dosing, milk levels ranged from unmeasurable to 30 units/L after a single dose and averaged 50 units/L after multiple doses.

A single IM dose of penicillin G was administered to 10 women. Milk levels were about 60 units/L and relatively constant for 6 hours after a 200,000-unit dose in 4 women. At 2 to 4 hours after a 500,000-unit dose in 4 women, milk levels reached 240 units/L. Peak levels of 240 to 360 units/L occurred at 4 hours after a 600,000-unit dose in 2 women.

After 2 IM doses (2 million units/dose) of penicillin G in 15 women, peak and trough milk levels were 120 and 10 units/L, respectively. After 2 IM doses (4 million units/dose) in 5 women, peak and trough milk levels were 220 and 30 units/L, respectively. Peak levels occurred 3 to 6 hours postdose.

After an IM dose of 360 mg (about 550,000 units) of penicillin G in 2 women, milk levels averaged 0.35 mg/L (530 units/L) at 1 hour, 0.2 mg/L (300 units/L) at 2 hours, and 0.1 mg/L (150 units/L) at 4 hours after dosing; at 6 hours postdose, only a trace was detectable.

A Herxheimer reaction occurred in a 1-month-old breastfed infant with congenital syphilis 6 hours after a 2.4 million unit IM dose of benzathine penicillin G was administered to the mother; the infant had also received 10 units of penicillin G at about the time of the maternal dose. The reaction was possibly due to penicillin in breast milk.

Disruption of infant's gastrointestinal flora (resulting in diarrhea or thrush) reported occasionally with penicillins; such effects have not been adequately evaluated.

See references