The balance of current evidence and clinical experience, and the consensus of specialist opinion, is that there is no established mutagenic or carcinogenic risk to infants breastfeeding from mothers receiving routine short-course treatment with metronidazole by any route.
Low-dose oral metronidazole, 200-400 mg three times daily, produces milk levels only slightly lower than corresponding levels in maternal plasma (76 to 99%). Doses up to 500 mg three times daily for a 7 to 10 day course are considered to be compatible with breastfeeding.
Single, 2 g high-dose oral metronidazole produces significantly higher levels in milk than low-dose oral therapy. The estimated total ingestion is 25.3 mg of metronidazole by the infant after feeding for 48 hours, although this is still lower than the daily infant dose given directly. If feeding is delayed for 12 hours after each dose, total infant exposure is reduced. Daily 2 g oral doses, normally given for 3 days, are considered to be compatible with breastfeeding, and in practice, delaying breastfeeding for 12 hours is not considered necessary.
Intravenous administration produces similar maternal plasma and milk levels to equivalent oral doses, although the data are limited. Short-course IV metronidazole is considered to be compatible with breastfeeding.
Administration of metronidazole by rectal, vaginal, or topical routes produces significantly lower plasma levels, and would therefore be expected to produce correspondingly lower milk levels than after oral administration, and is considered to be compatible with breastfeeding.
Side-effects in breastfed infants whose mothers have been treated with metronidazole are rare and unsubstantiated, and include loose stools, candidiasis and lactose intolerance. Anecdotal reports of infants rejecting milk at the start of feeds may be due to a metallic/bitter taste imparted to foremilk by a water soluble metabolite, although this again has not been substantiated.
There are no data relating to the effects of metronidazole exposure in preterm breastfed infants. Special consideration should be given to the use of metronidazole by any route in preterm or low-birth-weight infants, or in infants with compromised renal or hepatic function.