My dog swallowed a Metronidazole 400mg tablet. Will she be okay?

Oh no, it's not good that Bella ate the metronidazole tablet! There is a wide range of doses for metronidazole for dogs depending on the condition being treated, and at her weight and the dose she ate, the tablet is within the range of doses. It's on the higher end, but still within the dose range. That being said, you could see some side effects from eating this medication, such as GI upset. Excessive salivation, gagging, nausea, vomiting, and decreased appetite are the most common side effects seen. If you see any of those signs, withhold food for 8-12 hours, then feed her a bland diet of boiled white meat chicken (no bones) and white rice in small amounts several times a day for a couple of days. Then slowly add back in her normal diet. Rarely you can see neurological disorders from an accidental overdose, but she likely may not show these signs due to her not overdosing. It's just something I will mention you should be aware of, and if you see anything concerning or she won't stop vomiting, won't eat the bland diet, and/or becomes very lethargic, take her into a vet right away. Good luck, I hope this helps!