If you are only treating one dog I suggest panacur from the vet, that way you're getting the proper dosage.
Metronidazole: The drug itself has been around for many years and properly administered by a veterinarian
D.V.M. most dogs do just fine on it. My dogs have never had a bad reaction to metronidazole but it was administered by a vet and I carefully followed the instructions in giving it to my dogs. Per the vets I've spoken with the side effect problems often have to do with incorrect dosages.
Metronidazole is not 100% effective against giardia.
Giardia | CAPC Vet
It does, however, have a soothing and anti-inflammatory effect on the GI tract and in combination with fenbendazole is a good combination when dosed properly under the care of a veterinarian.
I've also read/heard people go ballistic about metronidazole destroying the beneficial gut bacteria. I've found that most vets are prescribing bene-bac or forti-flora and advising ongoing probiotic supplementation to alleviate that issue.
I really think metro gets a bad rap because people mis-use it, over use it or don't follow instructions.