I have a 6-year-old suffering from a bout of what seems to be colitis. Stools have been negative for Giardia, etc. He was prescribed metronidazole but after 4 days, still having mucus in his stool and still soft stools. I was told to give him two 500-mg tablets (1000 mg) twice a day. In speaking with a friend of mine, she says she gives her dogs just 1 tablet twice a day. I am wondering of 1000 mg twice a day is too much. He is 90 pounds but he does seem to have a sensitivity to meds. One time he was prescribed 1500 mg Keflex twice a day for hot spots but could not tolerate that dose. I cut him down to 500 mg once a day and he did fine and the hot spots cleared up. TIA.

According to my notes I have on treating Giardia, using Metronidazole/Flagl the dose is 33 mg per pound of dog.

That would be 2970 mg for a 90 pound dog per day. Seems like alot to me. When you do a search on antibiotic dosages, it seems to vary as well as giving it in one dose or splitting it up into 2 doses, one for morning and one in the evening.

Best to ask your vet what he or she advises on dosage and how many times a day to give.

I have always been told by my vet to give 500mg 2 times per day, regardless of the reason.

From what I understood, metronidazole is an anti bacterial. I have used it for Ecoli, coliform, and interdigital cyst.

Not sure about that dosage of metronidazole but 1500 mg of Keflex twice daily is a very high dosage for a dog of that weight in my opinion. Maybe that's why he reacted and did fine at 500 mg. The most I've heard of for a dog of that size is 1000 mg. twice a day or 500 mg. 4 times daily. Could you give the metronidazole 4 times daily in smaller doses and should it be dosed with food?

Maybe he's not sensitive to medications, your vet might be agressively dosing him

Your post is just the right timing. My male was just diagnosed today with Clostridium perfringens colitis after we had a terrible night of hemorrhagic diarrhea. He is 84 lbs and is getting 1 500 mg tablet twice daily.

My girl had hemorrhagic gastroenteritis right on Easter. She spent the day in the hospital and came home on Flagyl 1000mg twice a day. Generally for less extreme illness, like Giardia, I'd give 500mg twice a day per my vet's instructions.
BTW, Flagyl is not an antibacterial, it's an antiflagellate, so it kills "bugs" like Giardia and other little swimming pests that are prone to attack to GI tract.