Zonisamide blood level in dogs and cats
Zonisamide is a medicine for seizures, used in dogs on its own or together with other antiepileptic medicines. Levels are not checked routinely, but the vet may order one when seizures are not well controlled or after the regimen has changed. The result is read together with the time of the last tablet and the other medicines the animal takes.
What it shows
The test measures how much zonisamide is in the blood. Its half-life in dogs is relatively long, about 18–28 hours, so the level settles (the steady state) within about a week. A check is described about 1–2 weeks after the start or after any change in what the vet prescribed; some labs suggest about two weeks.
Labs recommend a trough sample, taken shortly before the next tablet — one reference describes about an hour before it. Some labs also accept a sample about 2 hours after the tablet. For comparison over time, the same timing each time helps most.
Zonisamide is broken down in the liver by the same enzyme system that phenobarbital speeds up. So the same regimen can give a different level in an animal that also takes phenobarbital, which is one reason a vet may want to see the number.
Higher than reference — what it may be a sign of
- Above the target window. Side effects are usually mild — passing wobbliness, loss of appetite, lethargy and vomiting. Rarer effects described in dogs include liver problems, metabolic acidosis, urinary stones and effects on thyroid hormones. Because the medicine is related to sulfonamides, one reference also lists possible dry eye and bone-marrow changes, while another considers dry eye unlikely.
- Side effects do not follow the level closely. Adverse reactions are not directly tied to the serum value, so a level inside the window does not rule them out and a high one does not prove them.
- Timing. A sample after the tablet is higher than a trough.
A high result is not a signal to change anything at home: the vet decides what to do with it.
Lower than reference
A level below the window may go with poorer seizure control. Practical reasons include a sample taken before the steady state, missed tablets and phenobarbital given at the same time, which speeds up the breakdown of zonisamide. The sample matters too: hemolysis can affect the result, so labs ask for gentle collection and quick separation of serum or plasma, and separator gel tubes are not recommended.
Look at together with
- Phenobarbital — it lowers the zonisamide level, so both are read together.
- Total T4 — long-term use can affect thyroid hormones.
- Bicarbonate and the urine test — metabolic acidosis and urinary stones are among the rare described effects.
- Liver values (and the complete blood count, if the vet orders it) — rare liver problems are described in dogs, and a drop in blood cells in cats.
Units and conversion
Zonisamide is reported in µg/mL. In the app this row is kept in µg/mL; if the form uses another unit, the row keeps the unit printed on the form.
Reference range
The target range is on your lab’s form. No window has been worked out specifically for dogs or cats: labs use the human therapeutic range of about 10–40 µg/mL and consider it applicable to dogs. Because animals vary, a good response is seen in some dogs with levels above or below this range, so the target for a particular animal is set by the vet.
Cats and dogs
In cats the half-life is longer than in dogs, about 33–68 hours. Clinical evidence for zonisamide in feline epilepsy is limited. In cats a presumed hypersensitivity with enlarged lymph nodes and a drop in blood cells has been reported, and stomach and gut upset was seen with long, intensive use.
In dogs the level settles within about a week and is usually checked as a trough. If the dog also takes phenobarbital, the zonisamide level tends to be lower on the same regimen. During long use thyroid values may be followed alongside the level.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- Charalambous M. Epilepsy in Small Animals. Merck Veterinary Manual, professional version, full review October 2025.
- Castel A. Anticonvulsants in animals (pharmacology of the nervous system). Merck Veterinary Manual, professional version, full review April 2025.
- Cornell University Animal Health Diagnostic Center. Zonisamide — testing protocol.
- Michigan State University Veterinary Diagnostic Laboratory. Therapeutic Drug Monitoring for Antiepileptic Drugs in Dogs and Cats (newsletter, winter 2018).
- Texas A&M Veterinary Medical Diagnostic Laboratory. Therapeutic drug monitoring in dogs and cats.
- Auburn University College of Veterinary Medicine. Therapeutic Drug Monitoring (TDM): pricing and sample collection protocols (2023).
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.