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Dog on phenobarbital: What side effects are normal & when to worry?

Updated On September 23rd, 2025

Pet's info: Dog | Dachshund | Male | 22 lbs

My dog 1.5 year Dachshund has been on phenobarbital since last 4-5 days. I want to understand the side effects if any and what precautions should I take to make sure he is fine. Historically, His ALP is elevated. In the range of 500 Also he is a bit lethargic and sleepy since we started with the medicine. Is this normal and any symptom that I should be aware of that is not normal. somebody was telling me that his hind legs may lose stability. How long the side effects last. Thanks as always

2 Answers

Most Helpful Answer

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Answered By Amy S. Eutsey, DVM

Veterinarian

Published on March 18th, 2020

Hello. Typically, phenobarbital takes a bit of time to get used to. The average amount of time to see some effects is 4-6 weeks, but every animal is an individual case, so it can vary. Expected side effects would be drowsiness, excessive drinking and sometimes excessive hunger. If your pet is so drowsy that they cannot walk , eat or function relatively normally - I would suggest that you speak with your veterinarian about adjusting the dose. It can elevate liver enzymes. Usually, blood is drawn periodically to check the therapeutic levels. This helps determine dosage adjustments. If your pet has months in a row without a seizure, you may want to ask about lowering the dose. We try to achieve the lowest dose that is effective at controlling the seizure activity. Thank you for using Petco Pet Education Center, formerly Petcoach

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    Answered By Andrea M. Brodie, DVM

    Veterinarian

    Published on July 6th, 2018

    Unfortunately there is no one perfect initial dose for every dog. When we have a dog seizing frequently we start with a so-called starting dose. Phenobarbital is a strong barbiturate medication which does cause side effects such as lethargy, sleepiness, stumbling etc. until the dog is used to the medication, These side effects should get significantly less over the next 2 weeks. It may also take a while until the brain is sufficiently calmed down to reduce or stop the seizures, Unfortunately in older dogs seizures are often caused by a brain tumor and as it grows it will cause more seizures. Your dog will have to have a blood test after a while (I am sure your vet gave you a date for one) to check phenobarbital blood levels. If these levels are not within the therapeutic range, the dosage will need to be adjusted, It may take a few adjustments until the correct dose is achieved. The goal is, of course, to reduce seizures to either nil or, if that is not possible, to a number that is tolerable for dog and owner. Trust your vet. Every dog is different, every dog metabolizes the medication differently, some take longer to settle than others and others may be ok with the initial dosage. It is quite normal at this stage for your dog to have side effects. Should phenobarbital not be working in your dog, then there are a few alternative drugs that can be tried. But that is something that will happen down the road. Please let me know if you have any other questions or concerns and I would be more than happy to help you further. Please take the time to mark this question as helpful or not as it helps us to improve on the quality of our answers to other pet owners. Thank you for using Petco Pet Education Center, formerly Petcoach.

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