Zonisamide — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	5-10 mg/kg	q12h (every 12 hours)	Duration: Chronic therapy; adjust based on serum levels and clinical response Notes: Start at lower end and titrate upward. Therapeutic serum concentration: 10-40 µg/mL. Monitor for adverse effects.
Cat	PO	5-10 mg/kg	q24h (every 24 hours) initially, may increase to q12h	Duration: Chronic therapy; adjust based on response Notes: Cats have a longer half-life; start with once daily. Monitor for sedation and hepatic effects.
Horse	PO	10-20 mg/kg	q8h (every 8 hours) or q12h	Duration: Not well established; use with caution Notes: Limited data; short half-life may require frequent dosing. Monitor for adverse effects.

Clinical Indications & Species Uses

General Indications

Anticonvulsant for seizure disorders in companion animals

Dog (Canine)

Adjunctive therapy for idiopathic epilepsy
Monotherapy for epilepsy (off-label)
Treatment of refractory seizures
Seizure control in dogs with brain tumors (off-label)

Cat (Feline)

Adjunctive therapy for epilepsy (off-label)
Seizure control in cats with structural epilepsy (off-label)

Pharmacology & Mechanism of Action

Drug Class: Antiepileptic Drug (AED) | Pharmacological Group: Sulfonamide derivative (with anticonvulsant properties)

Mechanism of Action: Zonisamide is a sulfonamide derivative that exerts its anticonvulsant effects through multiple mechanisms. It blocks voltage-sensitive sodium channels and T-type calcium channels, thereby stabilizing neuronal membranes and reducing repetitive neuronal firing. Additionally, it enhances GABAergic neurotransmission and modulates dopaminergic and serotonergic systems. It also acts as a weak carbonic anhydrase inhibitor, which may contribute to its antiepileptic activity.

Pharmacodynamics: Zonisamide reduces seizure frequency and severity by suppressing abnormal neuronal discharges. It has a broad spectrum of activity against partial and generalized seizures. In dogs, it is effective as an adjunctive therapy for idiopathic epilepsy and may be used as monotherapy. Its effects are dose-dependent, and therapeutic serum concentrations are typically 10-40 µg/mL in dogs.

⚡ Pharmacokinetics Summary

Absorption: Zonisamide is well absorbed after oral administration in dogs and cats. In dogs, bioavailability is nearly 100%. Peak plasma concentrations occur within 1-4 hours after dosing. Food may delay absorption but does not significantly affect overall bioavailability.

Distribution: Zonisamide is widely distributed throughout the body, with a volume of distribution of approximately 1.5 L/kg in dogs. It crosses the blood-brain barrier and achieves cerebrospinal fluid concentrations similar to plasma. It is approximately 40-60% bound to plasma proteins in dogs.

Metabolism: Zonisamide is extensively metabolized in the liver, primarily via cytochrome P450 enzymes (CYP3A4 in humans; in dogs, similar pathways). The major metabolic pathways include reduction, acetylation, and glucuronidation. In dogs, the half-life is relatively long (approximately 15-20 hours), allowing twice-daily dosing.

Excretion: Zonisamide and its metabolites are excreted primarily in the urine (about 70-80%), with the remainder in feces. In dogs, renal clearance is significant, and dose adjustments may be needed in patients with renal impairment.

Half-Life: Dog: 15-20 hours; Cat: approximately 30-40 hours; Horse: approximately 2-4 hours (short half-life, requires frequent dosing)

Bioavailability: Dog: ~100% oral; Cat: ~100% oral (assumed); Horse: not well documented

Protein Binding: Dog: 40-60%; Cat: approximately 50%; Human: 40-60%

Available Formulations & Strengths

Oral Capsule 25 mg, 50 mg, 100 mg (PO)

Oral Tablet 25 mg, 50 mg, 100 mg (PO)

Oral Suspension (compounded) Various (e.g., 50 mg/mL) (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to zonisamide or sulfonamides
Severe hepatic impairment
Severe renal impairment (use with caution)
Pregnancy and lactation (unless benefits outweigh risks)

Warnings & Clinical Precautions:

Use with caution in patients with hepatic or renal disease; dose adjustment may be necessary.
May cause metabolic acidosis due to carbonic anhydrase inhibition; monitor acid-base status in long-term therapy.
In cats, monitor for signs of hepatic toxicity (anorexia, vomiting, icterus).
Abrupt discontinuation may precipitate withdrawal seizures; taper dose gradually.
May cause sedation or ataxia, especially at initiation or dose increases.
Use with caution in patients with a history of urolithiasis (calcium-containing stones).
Safety in pregnant or lactating animals has not been established; use only if clearly needed.

Adverse Effects & Reactions

Common:

Sedation
Ataxia
Lethargy
Vomiting
Diarrhea
Anorexia

Serious / Severe:

Hepatotoxicity (especially in cats)
Metabolic acidosis
Renal tubular acidosis
Blood dyscrasias (rare)
Severe skin reactions (e.g., Stevens-Johnson syndrome, rare)

Rare:

Pancreatitis
Urolithiasis
Hypothyroidism (long-term)
Idiosyncratic reactions

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Phenobarbital	Phenobarbital may increase the metabolism of zonisamide, reducing its half-life and serum concentrations. Dose adjustment may be needed.	Moderate
Potassium bromide	Additive anticonvulsant effects; may increase risk of sedation and gastrointestinal upset.	Mild
Carbonic anhydrase inhibitors (e.g., acetazolamide)	Additive risk of metabolic acidosis and urolithiasis.	Moderate
Cimetidine	May decrease zonisamide metabolism, increasing serum levels and risk of toxicity.	Moderate
Other CNS depressants (e.g., opioids, benzodiazepines)	Additive sedation and ataxia.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Severe sedation
Ataxia
Coma
Respiratory depression
Metabolic acidosis
Hypotension
Seizures (paradoxical)

Emergency Treatment Protocol: Treatment is primarily supportive. Induce emesis if recent ingestion and patient is conscious. Administer activated charcoal to reduce absorption. Provide intravenous fluids to correct acidosis and maintain hydration. Monitor vital signs, ECG, and neurological status. In severe cases, consider hemodialysis (zonisamide is dialyzable). Symptomatic and supportive care is essential.

Food Animal Withdrawal Times

Zonisamide is not approved for use in food-producing animals. Withdrawal times are not established. Use in food animals is prohibited or requires extreme caution and extended withdrawal periods (e.g., >30 days for meat and >7 days for milk) as per veterinary discretion and regulatory guidelines.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C or 68-77°F).

Handling & Special Conditions: Protect from moisture. Keep in tightly closed container. Compounded suspensions may require refrigeration and have shorter shelf life; follow compounding pharmacy instructions.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (Zonegran) and used extra-label in animals.

Extra-Label (Off-Label) Use: Zonisamide is not FDA-approved for veterinary use; it is used extra-label in dogs and cats. In the US, extra-label use of approved animal and human drugs is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) for non-food animals. For food animals, extra-label use is prohibited if an approved animal drug is available and effective, and requires a valid veterinary-client-patient relationship and extended withdrawal times.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Zonisamide is a valuable adjunctive antiepileptic drug for dogs, particularly for those with refractory epilepsy. It is generally well-tolerated, with a favorable safety profile compared to some older AEDs. In cats, it is used off-label but requires careful monitoring for hepatic toxicity. Therapeutic drug monitoring is recommended to maintain serum concentrations within the target range (10-40 µg/mL in dogs). Dosing should be individualized, and gradual titration is advised to minimize adverse effects. Concurrent use with phenobarbital may require dose adjustments. In horses, its use is limited due to short half-life and lack of data. Always consider potential drug interactions and monitor for metabolic acidosis during long-term therapy. For food animals, zonisamide is not recommended due to lack of withdrawal data and regulatory restrictions.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)