I believe AAP states that Flagyl should be used with caution in BF mothers.
THis stems from some old research that it caused carcinogenic changes in the
liver of rats. This hs not been found in humans.

Dose can be 2g stat or 200-400mg tds.Theoretically may cause baby to reject
milk because it gives mik an unpleasant taste but in my experience they just
take it for granted normally. Peak levels in milk after 2-4 hours, after 12
hours only 20% remains. Dose received via milk is below that given directly to
children.

Full information in Medications and Mothers Milk (what less would we expect)

Wendy Jones
Breastfeeding Network UK
Pharmacist