What Are the Side Effects of Seizure Medication in Dogs?

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-14

Key Takeaways

• Most common initial side effects of seizure medication in dogs, such as drowsiness, increased thirst, increased urination, and appetite changes, are typically transient and resolve within 2-4 weeks as the body adjusts to the therapeutic drug levels.
• Certain severe adverse reactions necessitate immediate emergency veterinary intervention, including collapse, respiratory distress (wheezing, cyanotic gums), facial swelling, or seizures exceeding five minutes, as these can indicate anaphylaxis or status epilepticus.
• Phenobarbital, a common first-line anti-epileptic drug, requires regular monitoring (every 6-12 months) for potential liver enzyme elevation and toxicity, necessitating blood tests to assess drug concentration and organ function.
• Newer anti-epileptic drugs like levetiracetam generally exhibit fewer long-term side effects, particularly concerning sedation and hepatic impact, though renal function should be considered due to its primary elimination pathway.
• Abrupt cessation of anti-epileptic medication is contraindicated due to the risk of withdrawal seizures or status epilepticus; any medication adjustments or discontinuations must be guided by veterinary consultation.
• Consistent administration of prescribed doses at regular intervals is critical for maintaining therapeutic drug levels, preventing breakthrough seizures, and minimizing the risk of adverse effects, with a seizure log being an essential tool for monitoring efficacy and side effects.

If your dog has started seizure medication, you are likely watching closely for any change in behavior. Most dogs tolerate these drugs well, but side effects are common, especially in the first few weeks. The good news is that most side effects are temporary and manageable. The most frequent ones are drowsiness, increased thirst, increased urination, and a larger appetite. These usually fade as your dog's body adjusts, often within two to four weeks.

However, some side effects require immediate attention. If your dog collapses, has trouble breathing, develops hives or facial swelling, or has a seizure that lasts longer than five minutes, stop reading and go to the nearest emergency veterinary hospital now. These signs can indicate a serious allergic reaction or a life-threatening seizure emergency. Do not wait to see if they improve.

This guide explains what you can expect from common anti-epileptic drugs, how to tell a normal adjustment period from a problem, and when to call your veterinarian. You know your dog better than anyone. Trust your instincts if something feels wrong.

Is This an Emergency?

Use this table to decide your next step. When in doubt, call your veterinarian. They can tell you whether to come in or monitor at home.

{
 "type": "triage",
 "title": "Urgency triage by severity of signs",
 "tiers": [
  {
   "tier": "danger",
   "label": "Go to an emergency vet now",
   "signs": [
    "Seizure lasting over 5 minutes, or multiple seizures in 24 hours",
    "Collapse or fainting",
    "Difficulty breathing, wheezing, or blue or pale gums",
    "Swelling of the face, muzzle, or eyelids"
   ],
   "action": [
    "Call ahead to say you are coming",
    "Do not give any more medication",
    "Bring the medication bottle with you"
   ]
  },
  {
   "tier": "warn",
   "label": "Book a same-day appointment",
   "signs": [
    "Extreme lethargy where your dog will not get up to eat, drink, or go outside",
    "Stumbling, wobbling, or falling that is getting worse after the first week",
    "No appetite for more than 24 hours",
    "Vomiting more than once in a day"
   ],
   "action": [
    "Call your regular clinic as soon as they open",
    "Describe exactly what you have seen and when it started"
   ]
  },
  {
   "tier": "safe",
   "label": "Watch at home for 48 hours",
   "signs": [
    "Mild drowsiness or sleepiness in the first week",
    "Drinking and urinating more than usual",
    "Increased hunger or begging",
    "Mild wobbliness in the back legs that improves each day"
   ],
   "action": [
    "Keep a daily log of how often your dog drinks, urinates, and sleeps",
    "Call your vet if signs worsen or do not improve after two weeks"
   ]
  }
 ]
}

What Is Happening Inside Your Dog's Body

Seizure medications work by calming the electrical activity in the brain. Drugs like phenobarbital and potassium bromide are the older, well-studied options. Newer drugs such as levetiracetam, often sold under the brand name Keppra, work differently and tend to have fewer long-term side effects.

The drowsiness your dog feels is the medication slowing down brain activity. This is expected. Think of it as the drug doing its job. The increased thirst and urination happen because some of these drugs affect how the kidneys process water and salt. The increased appetite is a direct effect on the brain's hunger center. None of these are dangerous on their own, but they can be disruptive to your daily routine.

Your dog may also seem uncoordinated, especially in the back legs. This is called ataxia. It often appears in the first week and then improves. If ataxia gets worse instead of better, or if your dog cannot stand at all, that is a reason to call your vet the same day.

Phenobarbital: The Most Common First Choice

Phenobarbital is the most widely prescribed seizure medication for dogs. It is effective and inexpensive, which is why many veterinarians start with it. The side effects are well documented because the drug has been used for decades.

In the first two weeks, expect sleepiness, increased thirst, increased urination, and a ravenous appetite. Your dog may drink twice as much water as before. You will likely need to take them out for bathroom breaks more often. Some dogs gain weight during this period because they eat more. You can manage this by measuring food portions strictly and choosing low-calorie treats like green beans or carrot pieces.

Long-term use of phenobarbital can affect the liver. Your veterinarian will want to run blood tests every six to twelve months to check liver enzyme levels. This is routine monitoring, not a sign that something is wrong. The FDA Center for Veterinary Medicine provides information on approved animal drugs and their safe use, including the importance of regular blood work for medications like phenobarbital.

A rare but serious side effect is liver damage. Signs include yellowing of the gums or eyes, vomiting, loss of appetite, and extreme lethargy. If you see these, call your vet immediately. Do not stop the medication on your own, as sudden withdrawal can trigger seizures.

Levetiracetam and Other Newer Options

Levetiracetam, known as Keppra, is often used when phenobarbital is not effective or causes too many side effects. It is also used in combination with other drugs. The main advantage is that it causes less sedation and has fewer effects on the liver.

The most common side effects of levetiracetam are mild drowsiness and weakness, especially in the first few days. Some dogs experience diarrhea or a decreased appetite. These effects are usually short-lived and milder than those from phenobarbital. Because levetiracetam is removed from the body through the kidneys, dogs with kidney disease may need a different dosing schedule. Your veterinarian will determine the correct dose based on your dog's weight, health status, and blood test results. Never adjust the dose yourself.

Other drugs your vet might prescribe include zonisamide, which can cause drowsiness and loss of appetite, and potassium bromide, which can cause stomach upset and sedation. Potassium bromide can also cause a dry cough in some dogs. Each drug has its own profile, and your veterinarian will choose based on your dog's specific seizure type and overall health.

How to Give Medication Safely

Giving seizure medication correctly is critical. Missing a dose can trigger a seizure. Giving too much can cause dangerous side effects. The FDA's guidance on giving pet medication safely emphasizes checking the label every time, using the measuring device that came with the medication, and never guessing at a dose.

Always give the exact amount your veterinarian prescribed. Do not split pills unless your vet tells you they are scored for splitting. If your dog spits out a pill or vomits within an hour of receiving it, call your vet for instructions. Do not give a second dose without asking.

Keep a calendar or set a phone alarm for medication times. Consistency matters more than the exact hour, but try to stay within the same two-hour window each day. If you miss a dose, give it as soon as you remember, unless it is almost time for the next dose. In that case, skip the missed dose and continue on schedule. Never give a double dose.

When to Call Your Veterinarian

You should call your vet if any side effect lasts longer than two weeks, if a mild side effect suddenly becomes severe, or if you see any new symptom after the first month of treatment. The American Veterinary Medical Association's pet owner resources advise that any change in your pet's behavior or health after starting a new medication deserves a phone call.

Also call if your dog's seizures become more frequent or more severe. This can mean the medication dose needs adjustment or the drug is not working. Your veterinarian may recommend blood tests to check the drug level in your dog's system. This is called therapeutic drug monitoring, and it helps ensure your dog is getting enough medication to control seizures without reaching toxic levels.

Do not stop giving the medication without veterinary guidance. Abruptly stopping an anti-epileptic drug can cause withdrawal seizures, which can be worse than the original problem. If you are concerned about side effects, talk to your vet about options. There are often alternative drugs or dose adjustments that can help.

What You Can Do at Home Right Now

If your dog is on seizure medication and showing side effects, start by staying calm and observing. Most side effects are manageable, but a few need immediate hands-on care.

You must not handle these situations remotely:

• Your dog collapses, cannot stand, or is unresponsive
• Your dog has a seizure lasting more than 5 minutes, or multiple seizures without waking up between them
• Your dog is vomiting repeatedly, has diarrhea with blood, or cannot keep water down
• Your dog's gums look pale, white, blue, or gray
• Your dog is breathing fast at rest, more than 40 breaths per minute, or seems to be working hard to breathe
• Your dog shows signs of a severe allergic reaction, such as facial swelling, hives, or sudden weakness

Any of these require immediate in-person emergency care. Do not wait for a video call. Do not try to treat these at home.

What you can safely do at home:

• Keep your dog quiet and comfortable in a cool, dim room
• Offer small amounts of water frequently, but do not force drinking
• Feed a bland meal only if your dog is eating normally and your vet has already approved it
• Do not give any extra medication, including over-the-counter products, unless your veterinarian specifically told you to
• Do not stop or reduce the seizure medication on your own. Stopping suddenly can trigger withdrawal seizures or a dangerous condition called status epilepticus, where seizures do not stop

What to Write Down Before You Call

Before contacting your veterinarian, gather concrete information. Write down:

• The exact medication name and strength printed on the bottle
• The dose you give and the time of day you give it
• When the side effect started, and whether it began after a dose change
• How many times in the past 24 hours your dog has vomited, had diarrhea, or seemed unsteady
• Whether your dog is eating and drinking normally
• Your dog's weight, if you have a recent number
• Any other medications, supplements, or flea and tick products your dog receives

This list helps your vet decide whether an adjustment, a blood test, or an in-person visit is needed.

Side Effect Comparison Table

Different seizure medications cause different side effects. This table shows what owners commonly see.

Drowsiness is the most common side effect across all of these drugs. It often improves within 2 to 3 weeks as your dog's body adjusts. If it does not improve, or if it gets worse, call your vet.

The Monitoring Checklist

Your veterinarian will want to track your dog's response to treatment. Keep a simple log with these items:

• Seizure frequency: How many seizures, and on what dates
• Seizure duration: How long each seizure lasts, in minutes
• Recovery time: How long until your dog seems back to normal
• Appetite: Normal, reduced, or increased
• Water intake: Roughly how many bowls per day
• Urination: Normal frequency or noticeably more
• Energy level: Normal, sleepy, or lethargic
• Gait: Normal, mild wobble, or unable to walk steadily
• Vomiting or diarrhea: How many episodes per day
• Weight: Weekly, if possible

Bring this log to every veterinary visit. It gives your vet the data needed to adjust dosing safely. Dosing for seizure medications is highly individual. It depends on your dog's weight, liver and kidney function, and blood levels of the drug. Only your veterinarian can determine the right dose, and blood tests are often needed to guide it. The FDA emphasizes that giving pet medication safely means following the label and your vet's instructions exactly, and never guessing at a dose.

What a Veterinary Visit Usually Involves

When you bring your dog in for a side effect concern, the veterinarian will start with a full history. Expect questions about seizure timing, appetite, thirst, energy, and any changes since the medication started.

The physical exam will include listening to the heart and lungs, feeling the abdomen, checking hydration, and watching your dog walk. The vet may recommend bloodwork. For phenobarbital, this often means checking liver enzyme levels and the drug concentration in the blood. For other medications, kidney values and electrolyte levels may be checked.

The cost of this visit varies widely by region and clinic. A basic exam with bloodwork might range from roughly $150 to $400, but this is only general context. Prices differ based on your location, the clinic type, and whether your dog needs additional tests like imaging. Ask for an estimate before any testing.

Follow-up visits are common. Many dogs need blood levels checked a few weeks after starting a medication, then every 6 to 12 months after that. The American Veterinary Medical Association provides guidance on working with your veterinarian to plan ongoing care and understand what each test is for.

When an Online Vet Can Help, and When It Cannot

The situations in this article that require hands-on care are clear. A dog with an active seizure, collapse, breathing trouble, or signs of a severe allergic reaction must go to an emergency clinic immediately. A dog that is vomiting repeatedly or cannot keep water down also needs in-person care. These are not video call situations.

A remote consultation is genuinely useful for other scenarios. If your dog is mildly drowsy in the first week of a new medication, an online vet can help you decide whether that is expected. If you are unsure whether a single episode of vomiting warrants a visit, a video call can clarify. If your dog has been stable for months and you have a question about a subtle change, remote care can save you a trip.

A remote veterinarian can take a detailed history, watch your dog move on video, and offer advice. In many states, they can prescribe medication. But they cannot palpate the abdomen, listen to the chest, take a temperature, run bloodwork, or image anything. Those require an in-person visit. Prescribing rules differ by state, and a veterinarian may require an in-person examination before prescribing or refilling a seizure medication. Use remote care as a triage tool, not a replacement for hands-on diagnosis.

Common Mistakes Owners Make With Seizure Medication

Mistake: Stopping the medication suddenly because the dog seems fine. Seizure drugs control the condition, they do not cure it. Stopping suddenly can trigger withdrawal effects or a cluster of seizures that is harder to control. If you believe the medication is causing problems, call your veterinarian before changing anything. They will guide you through a safe taper if a change is needed.

Mistake: Giving a double dose after a missed one. If you forget a dose and remember within a few hours, give it. If it is almost time for the next dose, skip the missed one and continue the normal schedule. Never give two doses at once. Doubling up can cause heavy sedation, wobbliness, or worse. The FDA's guidance on giving pet medication safely stresses measuring carefully and following the label exactly.

Mistake: Assuming drowsiness will pass quickly. Many owners expect the sleepiness to vanish in a few days. With phenobarbital, it can take two to three weeks for the body to adjust. With levetiracetam, sedation is often milder but can still last a week or more. Give the adjustment period time before judging the drug.

Mistake: Hiding pills in food without checking if the dog ate it. A dog that spits out half a pill may be underdosed for days. Give the medication directly, then offer a small treat. Watch the dog swallow. If you use a pill pocket, confirm the whole dose went down.

Mistake: Ignoring changes in thirst or appetite. Increased drinking and urination are common with phenobarbital, but extreme thirst can signal a problem. If your dog drinks more than double their normal amount, or starts having accidents in the house, tell your veterinarian. Do not restrict water, dehydration is a separate risk.

How to Reduce the Chance of It Happening Again

Seizure medication side effects are not fully preventable, but you can lower the risk of serious problems.

Use one pharmacy and one veterinarian. Keeping a single record of every prescription reduces the chance of drug interactions or dosing errors. The American Veterinary Medical Association recommends keeping a current list of all medications your dog takes, including supplements.

Test blood levels as directed. Phenobarbital levels need checking after starting the drug, after any dose change, and then every six to twelve months. These blood tests catch toxicity before you see signs. Levetiracetam usually does not need level monitoring, but your vet may still want periodic blood work.

Keep a steady routine. Give doses at the same times each day, roughly twelve hours apart. A consistent schedule keeps drug levels stable, which reduces breakthrough seizures and side effect spikes.

Store medication properly. Keep drugs in their original containers, away from heat and moisture. Check expiration dates. A degraded drug may not work as expected.

Do not add supplements without asking. Some herbal products and supplements can interact with seizure drugs. Ask your veterinarian before adding anything, even over-the-counter options.

What Recovery and Ongoing Management Usually Look Like

Most dogs adjust to seizure medication within two to four weeks. The first week is often the hardest. Your dog may sleep more, seem unsteady, or act slightly dull. These signs usually fade as the brain adapts.

For phenobarbital, expect the sedation to improve gradually over two to three weeks. Some dogs keep a mild calmness long term, which is normal. Increased thirst and urination often persist, so plan for more frequent bathroom breaks.

For levetiracetam, side effects are typically milder and shorter. If your dog is still very sleepy after two weeks, tell your veterinarian. The dose may need adjusting or a different drug may suit your dog better.

Full control of seizures can take time. Your veterinarian may adjust the dose several times in the first few months. Blood tests guide these changes. Do not expect perfection immediately. The goal is fewer, shorter, and less frequent seizures, not necessarily zero.

Long term, your dog will need regular checkups, usually every six months. These visits include blood work to check liver function and drug levels. Most dogs on seizure medication live normal, happy lives with this monitoring in place.

Frequently Asked Questions

Will my dog be sleepy forever? Usually not. The drowsiness is strongest in the first one to three weeks. After that, most dogs settle into a normal energy level, though some remain slightly calmer than before. If your dog is still very sleepy after three weeks, call your veterinarian. The dose may be too high, or the drug may not be the best fit. Blood tests can check whether the level is in the target range.

Can I give my dog a human seizure medication? No. Human formulations may contain different ingredients or release the drug at a different rate. Dosing is also weight-specific and must be determined by a veterinarian. Giving the wrong product can cause toxicity or ineffective seizure control. Always use the medication prescribed for your dog and follow the label exactly.

What if my dog vomits after taking the pill? If vomiting happens within thirty minutes of the dose, call your veterinarian. They may advise giving another dose or waiting. If your dog vomits repeatedly, seek care. Repeated vomiting can mean the drug level is too high or that your dog has a separate stomach problem. Do not give another dose without guidance.

How long does it take to know if the medication works? Seizure drugs build up in the body over days to weeks. Phenobarbital takes about two weeks to reach steady levels. Levetiracetam works faster, often within days. Your veterinarian will usually wait at least two weeks before judging effectiveness. Keep a seizure diary during this time so you can report the frequency and length of any seizures.

Is it safe to board my dog while on this medication? Yes, with planning. The boarding facility needs written instructions and enough medication for the stay. Give them your veterinarian's contact information. Ask them to confirm your dog is eating and drinking normally. If your dog has a seizure while boarded, staff should know your emergency plan and who to call.

Can my dog still go for walks and play? Yes, once the initial sedation passes. Start with short walks and watch for wobbliness. Avoid swimming, off-leash running near roads, or any activity where a sudden seizure could be dangerous. As your dog stabilizes, most normal activities can resume. Always tell anyone caring for your dog about the condition and what to do during a seizure.

Related Clinical & Scientific Guides

• What Are the Side Effects of Bravecto for Dogs? What to Watch For
• Can You Give Your Dog Human Allergy Medicine? Whats Safe
• Can You Give Your Dog Dramamine for Car Sickness?

References and Further Reading

• FDA Center for Veterinary Medicine Animal Drug Information
• FDA Safe Use of Pet Medications
• AVMA Pet Owner Resources
• MSD Veterinary Manual Pet Owner Version
• AAHA Pet Owner Resources

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This article is general educational information for pet owners. It is not a substitute for veterinary diagnosis, examination, or treatment. If your pet is showing severe or rapidly worsening signs, contact an emergency veterinary hospital immediately.