Generic Name: metronidazole
Brand Name: Flagyl

Metronidazole is an antibiotic used in both human and veterinary medicine to treat a variety of disease states, including chronic diarrhea.

Toxicity often occurs from intentional dosing at levels greater than 30 mg/kg/day. While much of the literature warns of potential toxicity at doses greater than 60 mg/kg/day, there is an increasing incidence of toxicity occurring at lower doses. Beyond dosing, there seems to be a positive correlation of prolonged therapy (3 weeks and longer) and toxicosis.

Signs and symptoms of toxicity: difficulty walking, rapid eye movements, head-tilt, joint knuckling, disorientation, refusal to eat, vomiting, extreme fatigue, stiffness and seizures.

Toxic consumption: The referenced toxic threshold is 60 mg/kg/day (values listed in chart below). As previously mentioned, chronic therapy with lower doses may result in toxicosis.

Please note that these values are total daily consumption.

Dogs: Metronidazole Toxic Consumption
X-Small Yorkie, Chihuahua	Small Pug, Boston Terrier, Poodle	Medium Beagle, Scottish Terrier	Large Boxer, Cocker Spaniel	X-Large Labrador & Golden Retrievers, German Shepherd	XX-Large Great Dane, St. Bernard
1 – 10 lbs. (0.45 – 4.6 kg)	11 – 25 lbs. (5 – 11.4 kg)	26 – 40 lbs. (11.8 – 18.2 kg)	41 – 70 lbs. (18.6 – 31.8 kg)	71 – 90 lbs. (32.3 – 40.9 kg)	91 – 110 lbs. (41.4 – 50 kg)
> 27 mg	> 300 mg	> 708 mg	> 1116 mg	> 1938 mg	> 2484 mg

References:
– Evans J, Levesque D, Knowles K, et al. Diazepam as a treatment for metronidazole toxicosis in dogs: A retrospective case study of 21 cases. J Vet Intern Med 2003;17:304-10.
– Metronidazole. In: Plumb’s Veterinary Drugs. Tulsa (OK): Educational Concepts, L.L.C. [updated 8/14; accessed 8/14/15]. https://www.plumbsveterinarydrugs.com/#!/monograph/2566
– Papich MG. Saunders Handbook of Veterinary Drugs: Small and Large Animal. 3rd ed. St. Louis, MS: Elsevier Saunders; 2011.