Here is what the Canadian equivalent of the PDR says about metronidazole: 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 diseases (amebiasis, giardiasis or trichomoniasis) for which metronidazole may be the treatment of choice. ‹Drug Exposure During Lactation. Compendium of Pharmaceutical Specialities. 1997. page L36. The issue is, once again, does a small amount of metronidazole in the milk render breastfeeding more hazardous than formula? The answer, in my book is patently no, and breastfeeding is *still* better for mother and baby. Jack Newman, MD, FRCPC