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We don't have any studies on the efficacy or safety of Macrobid in dogs. There are some people that have used it at 2 mg per lb body weight. I have not personally used it, so I wouldn't recommend it.
For UTI's, the best thing is to start some antibiotics. An exam and some antibiotics should be relatively inexpensive to get this treated correctly. If you need to wait a day or two, you can give some cranberry juice 2-3 times daily, as it has some ability to lessen the signs of a UTI. You could also give Aspirin for the discomfort. The Aspirin dose in dogs is 5 mg per lb body weight every 12-24 hours.
I hope this helps, let me know if you have any other questions.