Isoniazid Toxicity in Dogs | VETgirl Veterinary Continuing Education Podcasts

January 2024

In today’s VETgirl online veterinary CE podcast, Dr. Justine Lee, DACVECC, DABT discusses isoniazid toxicosis in dogs. While isoniazid isn’t a common toxicant seen in dogs, this antimicrobial potentially be fatal when dogs get into poisonous amounts. Isoniazid (commonly known as INH) is a human medication used for tuberculosis. While it is used in veterinary medicine to treat Mycobacterium or Actinomyces, it has a narrow margin of safety in dogs and cats.1,2 This drug works by blocking the synthesis of mycolic acid. INH depletes the central nervous system (CNS) of pyridoxine and also decreases levels of GABA within the brain. Many assume that since this is an “antibiotic” that it is safe; however, when accidentally ingested in dogs (and rarely, cats), it can result in severe CNS signs (e.g., tremors, refractory seizures, coma, death). The LD50 in dogs is estimated to be as low as 50 mg/kg;1,2 at this same dose, seizures can be seen. One 300 mg tablet can result in severe poisoning in a 10-pound dog. Other clinical signs include gastrointestinal signs (e.g., hypersalivating, vomiting, diarrhea), acid-base disturbances (e.g., metabolic acidosis), hyperthermia (secondary to tremors or seizures) and organ injury (e.g., hepatic injury, acute kidney injury, etc.). Due to the rapid onset of clinical signs, it is often too late to decontaminate the patient. Gastric lavage under anesthesia may be necessary. Treatment also includes IV fluids, anti-emetics, anticonvulsants, muscle relaxants, supportive care, and the antidote pyridoxine hydrochloride (typically available as 100 mg/ML) (Dose: suggested dose of 71 mg/kg IV, diluted to 5-10%, slow over 30-60 minutes).1,2 Clinicopathologic monitoring should include a biochemistry panel and recheck hepatic panel (3-5 days later). Vitamin B6/pyridoxine and prompt treatment and antidote therapy is necessary for best outcome. Veterinary professionals must be aware of the narrow margin of safety with this human and veterinary antibiotic.

1. Villar D, Knight MK, Holding J, et al. Treatment of acute isoniazid overdose in dogs. Vet Hum Toxicol 1995;37(5):473-7.
2. Schmid DR, Lee JA, Wismer TA, et al. Isoniazid toxicosis in dogs: 137 cases (2004-2014). J Am Vet Med Assoc 2017;251(6).

Only VETgirl members can leave comments. Sign In or Join VETgirl now!