Here is what the Compendium of Pharmaceutical Specialities (the Canadian
PDR) says about metronidazole in the Lilac Pages:

Metronidazole has been cited as a drug contraindicated during lactation.
This advice is based on reports that the drug is metagenic 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...

There it is in writing for those that like to have it that way.  It's been
in the CPS for several years.

Jack Newman, MD, FRCPC