Thank you. I believe that Louis has been overdosed and is showing the initial signs of overdosing which can include neurologic disorders, lethargy, weakness, anorexia, nausea, vomiting, and diarrhea. Peak blood levels arise ~1 hour after dosing. His increased respiratory rate suggests the discomfort you mentioned and that discomfort may currently be in his gastrointestinal tract.I recommend your doing two things for now. The first is administering PepcidTwo at a dose of 0.5 mg/kg of the famotidine in that product. That antacid should afford Louis some comfort. The second is skipping the next dose of metronidazole and then reducing the dose of metronidazole to 500 mg twice daily - the proper maximum dose for a 35 kg dog. I would expect Louis to remiss completely within 24 hours. If not, or, indeed, he worsens, please contact his vet to see if he/she wants to check Louis once again.Please respond with further questions or concerns if you wish.