Zonisamide for Canine Idiopathic Epilepsy: Efficacy, Tolerability, and Therapeutic Drug Monitoring

Bottom line

• Zonisamide (ZNS) is a newer-generation antiseizure medication used in canine idiopathic epilepsy; 59% of dogs achieve responder status on monotherapy per a 207-dog retrospective TDM cohort.1
• A prospective multicenter trial of 56 newly-diagnosed dogs reported that 76% achieved a >=50% reduction in seizure frequency and 55% achieved seizure freedom with monotherapy.2
• Adverse effects are generally mild and transient; 13% of dogs in the prospective study showed reduced activity, gastrointestinal signs, or hindlimb weakness, resolving without treatment discontinuation.2
• A therapeutic drug monitoring reference interval of 10-55 ug/mL has been proposed for dogs, broader than the human interval of 10-40 ug/mL.1
• ZNS is not FDA-approved for veterinary use in the United States; it is used off-label. Consult current formularies for species-specific guidance.

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What the evidence shows

Efficacy overview

Zonisamide is a sulfonamide-derived antiseizure medication that blocks voltage-gated sodium and T-type calcium channels, and may also enhance gamma-aminobutyric acid (GABA) activity. In human medicine it has been approved as an adjunct treatment for partial seizures; in veterinary medicine it is used off-label, primarily in dogs and to a lesser extent in cats.

A 207-dog retrospective cohort study covering a decade of therapeutic drug monitoring (TDM) data at Auburn University provides the largest efficacy dataset for ZNS monotherapy.1 Dogs were classified using the International Veterinary Epilepsy Task Force (IVETF) response categories: 59% of dogs (n=123) were responders, including 29% who achieved seizure freedom (category 1) and 30% who achieved partial response (category 2). Non-responders (category 3) comprised 41% (n=84). These figures came from dogs with non-structural epilepsy receiving ZNS monotherapy only.

A prospective multicenter open-label study in Japan evaluated 56 client-owned dogs with newly diagnosed idiopathic epilepsy (all ASM-naive, >=2 seizures in 12 weeks).2 Over 12 weeks of treatment, 76% (40/53 evaluable) achieved a >=50% reduction in seizure frequency, and 55% (29/53) achieved seizure freedom. These figures are consistent with what is reported for phenobarbital as first-line monotherapy in newly-diagnosed dogs, though direct comparative trials are limited.

Therapeutic drug monitoring

Therapeutic drug monitoring is considered important for optimizing zonisamide therapy. The 207-dog TDM cohort study proposed a reference interval of 10-55 ug/mL in dogs, which is notably broader than the accepted human range of 10-40 ug/mL.1 Responders achieved mean trough concentrations in the 18.9 ug/mL range in the prospective study, with a range of 8.0-48.0 ug/mL among those achieving >=50% seizure reduction.2 These data support the value of TDM for dose adjustment, though the retrospective design of the interval study limits causal inference.

Safety and tolerability

In the prospective 56-dog trial, adverse effects were observed in 7/56 dogs (13%) and included reduced activity, decreased appetite, vomiting, hindlimb weakness, soft stools, or constipation.2 Adverse effects were described as mild and temporary, and no dogs required discontinuation for safety reasons. Laboratory evaluations showed no clinically relevant changes in standard chemistry or haematology panels. This tolerability profile contrasts with phenobarbital, which carries known hepatotoxicity risk with chronic administration, and may make zonisamide a preferred option in dogs with concurrent hepatic concerns, although prospective head-to-head comparison data are limited.

Idiosyncratic reactions including hepatotoxicity and renal tubular acidosis have been reported in case series and are noted in review literature, though their frequency in the prospective and retrospective trial datasets was not quantified separately.

Clinical place

Zonisamide is typically used as a first-line or second-line monotherapy in dogs, and as adjunctive therapy in refractory cases. Its once- or twice-daily dosing and relatively predictable linear pharmacokinetics support clinical use, though individual variation in clearance means TDM is recommended for dose optimization. The proposed reference interval of 10-55 ug/mL1 provides a starting framework; doses should be adjusted based on clinical response and monitored trough concentrations, per current formulary and IVETF recommendations.

How this fits clinical practice

For practitioners managing canine idiopathic epilepsy, the evidence supports zonisamide as a viable monotherapy option, particularly in dogs where phenobarbital's hepatotoxicity risk is a concern. Establishing baseline bloodwork before starting and monitoring periodically is consistent with current practice norms. Therapeutic drug monitoring using the proposed 10-55 ug/mL reference interval assists dose titration. Consult current veterinary formularies such as Plumb's Veterinary Drug Handbook for specific dosing, monitoring intervals, and contraindication guidance; these parameters are beyond the scope of this evidence summary.

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References

1. Thungrat K, Jukier T, Boothe DM. Efficacy of monotherapy with zonisamide and proposed reference interval in dogs with epilepsy: a cohort of 207 dogs (2011-2021). J Vet Intern Med. 2026;40(1):aalaf026. https://europepmc.org/articles/PMC12881944
2. Saito M, Nomura A, Hasegawa D, et al. Clinical efficacy and tolerability of zonisamide monotherapy in dogs with newly diagnosed idiopathic epilepsy: Prospective open-label uncontrolled multicenter trial. J Vet Intern Med. 2024;38(4):2228-2236. https://europepmc.org/articles/PMC11256125

Changelog

• 2026-06-19: First published.

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