Nothing else works for Giardia, and Metronidazole and Flagyl only stops it for
a short time, then it returns. Plus dogs
that have the potential for
the MDR1 mutation gene (Collies, Shelties, Aussies, etc. with the merle gene should not be given Metronidazole or Flagyl).
Dogs with access to lakes, ponds, standing water are more prone to getting Giardia but if you have a problem with more than one dog and you have a well, I would certainly have your water tested for it.