I realize that others disagree with me, but I do not think that oral
metronidazole, even 2g as a single dose, contraindicates
breastfeeding.  Here is what the Compendium of Pharmaceutical
Specialities (Canadian physicians' desk reference) says:

"Metronidazole has been cited as a drug contraindicated during
lactation. This advice is based on reports that the drug is mutagenic
in bacteria and carcinogenic in rodents when taken throughout their
lives.  Specific untoward effects in a nursing infant as a result of
metronidazole ingestion have not been reported.  Without more direct
evidence of the harmful effects of short term use in humans it seems
overly conservative to withhold the drug or discontinue breastfeeding
in patients with symptomatic parasitic infections (amebiasis,
giardiasis, or trichomoniasis) for which metronidazole may be the
treatment of choice." page L38, thirty first edition 1996

Jack Newman, MD, FRCPC