With metronidazole, make sure you watch your dog closely for neurological problems. Metronidazole is prescribed regularly and generally well tolerated, but it nearly killed my Great Dane. He had a severe neurological reaction to it and had to spend a week at the neurologoist's office. When he came home, he still could not walk and had lost over 50 lbs. It was horribly scary and I didn't know if he would ever walk again.
So, with that being said, just watch for any strange neurological symptoms like staggering, twitching rear legs, nystagmas etc. When I worked at the vet clinic for a 5 year span, my dog was the only dog to ever react to it.