Phenobarbital blood level in cats and dogs
Phenobarbital is a long-used medicine for epilepsy in dogs and cats. The blood test does not look for disease: it measures how much of the medicine is in the blood, so the vet can see whether the level sits in the window where it works without causing harm. The result only makes sense together with the time of the last tablet and how the animal is doing.
What it shows
The effect of phenobarbital — both the wanted one and the side effects — follows its concentration in the blood, and the same medicine gives different levels in different animals. That is why vets measure it instead of relying on the number of tablets alone.
The level builds up slowly: it takes about 2–3 weeks to settle (the steady state). A commonly described schedule is a first test 2–3 weeks after the start or after any change in what the vet prescribed, another about three months later, and then every 6–12 months in a stable animal. An extra test is usual when seizures continue, when the animal is unusually sleepy, or when another medicine is added.
Most often blood is taken shortly before the next tablet — the lowest point of the day, called the trough. Some labs also describe a pair of samples: about two hours after the tablet (the peak) and shortly before the next one.
Higher than reference — what it may be a sign of
- Above the therapeutic window. Sleepiness and wobbliness are among the common side effects of phenobarbital; a high level makes them more likely.
- Liver risk in dogs. Long-term use is linked to liver damage, especially at high levels; this is why the level is followed together with liver tests.
- Timing of the sample. Blood taken soon after a tablet gives a higher number than a trough sample, so the time of the last tablet matters for reading it.
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 mean a higher risk of seizures coming back. Common practical reasons are a sample taken before the steady state was reached, missed tablets, another medicine added recently, and the sample itself: serum separator (gel) tubes can falsely lower the measured value, and hemolysis is best avoided. If the level is low while seizures are well controlled, the vet reads it together with how the animal is doing.
Look at together with
- ALP and ALT — in dogs phenobarbital itself often raises liver enzymes, especially ALP, without liver disease.
- Albumin, urea and bile acids before and after a meal — help the vet tell the effect of the medicine from real liver trouble.
- Complete blood count and biochemistry — part of regular monitoring in dogs.
- Total T4 — long-term phenobarbital can lower thyroid hormone values, rarely with real hypothyroidism.
Units and conversion
Phenobarbital is reported in µg/mL or µmol/L. One µg/mL is about 4.31 µmol/L, so 20 µg/mL is about 86 µmol/L. In the app each row keeps the unit printed on the form; the chart converts to µg/mL so forms from different labs line up.
Reference range
The therapeutic range is on your lab’s form. For phenobarbital it is not a range of healthy animals but a window in which the medicine usually works without toxic effects, and the lab prints its own. The Merck Veterinary Manual gives a target trough window of 15–35 µg/mL (about 65–151 µmol/L) for dogs and describes levels above 35 µg/mL as a particular concern for the liver; for cats, its epilepsy chapter gives 23–30 µg/mL, while another chapter of the same reference gives 15–35 µg/mL for both dogs and cats.
Cats and dogs
In cats phenobarbital does not tend to raise liver enzymes the way it does in dogs, so an increase in liver values in a cat on phenobarbital is more likely to point to real liver disease. Liver damage from the medicine is not described as typical for cats, but levels above 30 µg/mL have been linked to higher ALT, so periodic checks are still advised. The level settles a little faster in cats, with an average half-life of about 43 hours.
In dogs level checks are combined with liver tests every 6–12 months. ALP often rises because of the medicine itself, so the vet looks at albumin, urea and bile acids to tell drug effect from liver damage. The average half-life in dogs is about 53 hours, which is why the steady state takes weeks to reach.
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.
- Texas A&M Veterinary Medical Diagnostic Laboratory. Therapeutic drug monitoring in dogs and cats.
- Texas A&M Veterinary Medical Diagnostic Laboratory. Phenobarbital — test and specimen requirements.
- Michigan State University Veterinary Diagnostic Laboratory. Therapeutic Drug Monitoring for Antiepileptic Drugs in Dogs and Cats (newsletter, winter 2018).
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.