Isoniazid

Isoniazid is an antimycobacterial drug used primarily in human tuberculosis treatment. Veterinary therapeutic use is uncommon and confined to selected infections caused by susceptible mycobacteria, generally within specialist combination protocols. Susceptibility varies substantially among mycobacterial species, and published veterinary clinical evidence is sparse compared with human treatment experience.

Group

Antimycobacterial

Mechanism of Action

Isoniazid is a prodrug activated within susceptible mycobacteria, commonly by the catalase–peroxidase enzyme KatG. Activated intermediates form complexes that inhibit enzymes including InhA, disrupting synthesis of mycolic acids required for the mycobacterial cell envelope. Impaired envelope formation inhibits growth and can kill actively multiplying susceptible organisms. Differences in activation and target susceptibility help explain variation among mycobacterial species.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Selected mycobacterial infection	5 mg/kg	PO, IM, IV	Every 24 h	Initial daily regimen; published guidance allows escalation to 10-15 mg/kg/day.
Historical tuberculosis chemoprophylaxis	10 mg/kg	PO	Every 24 h	Human-dose extrapolation; treating M. tuberculosis or M. bovis in dogs/cats is not recommended in the supplied guidance.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Selected mycobacterial infection	5 mg/kg	PO, IM, IV	Every 24 h	Initial daily regimen; published guidance allows escalation to 10-15 mg/kg/day.
Resistant feline tuberculosis alternative	10–20 mg/kg	PO	Every 24 h	Second-line historical regimen only; consider organism identification, zoonotic implications and susceptibility.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Selected mycobacterial infection	10–20 mg/kg	IM	Every 24 h

Indications

Specialist combination treatment of selected infections caused by isoniazid-susceptible mycobacteria in veterinary patients; clinical efficacy evidence is limited and organism-specific.

Contraindications

Hepatic disease

Interactions

Itraconazole may decrease metabolism; rifampin may increase it.

Adverse effects

Hepatotoxicity, peripheral neuropathy and rash
Overdose-related seizures

Toxicity

Acute overdose is a neurologic emergency; anticonvulsant support and pyridoxine treatment may be required. Isoniazid overdose may require IV pyridoxine plus anticonvulsant treatment; its metabolic neurologic toxicity can be severe. Serious canine toxicity can follow ingestion of a single 300 mg tablet. Pyridoxine is described on a milligram-for-milligram basis relative to the known isoniazid amount, preferably IV over 30-60 minutes, together with seizure, acid-base and respiratory management; do not estimate from an unknown ingestion.

Preparations

Preparation	Form	Strength	Route
Isoniazid 100 mg tablet	Tablet	100 mg / tablet	PO
Isoniazid 300 mg tablet	Tablet	300 mg / tablet	PO
Isoniazid 10 mg/mL oral syrup	Oral Liquid	10 mg / mL	PO
Isoniazid 100 mg/mL injection	Injection	100 mg / mL	IV, IM

Monitoring and Administration

Monitor liver enzymes and neurologic signs.

Do not exceed 30 mg/kg in large animals.

Published guidance describes Johne disease treatment with rifampin but does not supply a complete route-specific combination protocol or evidence of reliable disease clearance.

Small-animal intermittent isoniazid alternative: 15 mg/kg two or three times weekly. Its route is not specified separately, and a calendar-based schedule is required.