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Phenobarbital

Written by: Maureen Setter

 

Table of Contents of frequently asked questions (FAQs)

 

 
 

 

 

Phenobarbital (PB), which is of the barbiturate family of drugs, is the most commonly prescribed antiepileptic drug (AED) for dogs. Because it is so commonly prescribed, its relative safety and efficacy (effectiveness) is well documented. It was first used in 1912 as the first synthetic AED for treating epilepsy (in humans). Phenobarbital is often referred to as simply 'pheno' or 'phenobarb' and abbreviated as PB.
 

Why is Pheno usually prescribed first?
Clinical reports indicate that 60 to 80% of epileptic dogs can be controlled effectively with only PB. It is relatively inexpensive, fairly easy to maintain PB serum levels with 2 or 3 times a day daily dosing and has few side effects. However, Potassium Bromide (KBr) is gaining ground on PB when it comes to being considered the first drug of choice when treating seizures.

But ...I'm afraid to use PB because of liver damage
In the beginning it is far more important to get the seizures under control than to be concerned about liver damage that "may" never occur. When PB is used properly, PB serum levels (discussed below) monitored as well as testing liver functions, the risk of liver disease is very low. If by chance your dog is beginning to experience early signs of liver disease, the condition can usually be reversed. The use of Milk Thistle can help to improve the liver.

What are the side effects?
If you notice any side effects, this will normally be in the first few weeks of starting PB therapy and also when the PB dosage is increased. These side effects will normally diminish in time. These are sedation, ataxia (loss of coordination), and the 3 P's: polydipsia (excessive thirst), polyuria (excessive urination) and polyphagia (excessive eating). Gaining weight is a common problem for many dogs on PB and this is one side effect that might not diminish. Therefore it is important to use as little as possible 'extra'food in giving medication. After long term usage, another problem is that the dog's system becomes use to the drug and will usually need a greater dosage to maintain control.

Where can I get PB?
PB is a controlled substance - but don't let that scare you. You must have a veterinarian's prescription to obtain it. You can get it either thru your veterinarian or pharmacy (with a prescription from your vet). Check around with the various pharmacies in your area and don't forget to check with the discount store pharmacies. PB is relatively inexpensive but it still doesn't hurt to shop around. PB comes in 15, 30, 60 (referred to as a grain) and 100 mg tablets and as a 4 mg/ml solution. If your vet prescribes 30 mg to be given every 12 hours, you'll find that it's easier to buy 30 mg tablets, but it will be less expensive to buy 60 mg tablets and split them. Since the actual cost of the drug is very inexpensive, the cost you pay is related more to the cost of filling the prescription than related to the cost of the medication.

Today all PB is generic. Occasionally your veterinarian or pharmacist will change pharmaceutical suppliers. If this happens you might notice a difference in the size/shape of the PB tablets. In general this should not present a problem for your dog. But just beware of the change. To be on the safe side also check the last bottle of PB with the new one to make sure the dosage is the same regardless of the tablet size, if any questions call your pharmacist or veterinarian.

What is a grain?
A grain is often referred to as 60 mg. The Merck Veterinary manual says 1 grain is 65 mg (approximately). Others often refer to a grain as 64 mg. The Webster dictionary says 1 grain is 64.8 mg and the Handbook of Chemistry & Physics says 1 grain is 64.798918 mg. The term comes from the weight of a grain of wheat.

How is the initial dosage determined?
Your veterinarian will take into consideration several factors when determining the initial starting dosage for your dog. These can include such things as: severity of your dog's seizures, frequency of seizures, weight of your dog and also other medications that your dog might be on.

When considering just your dog's weight, the initial daily dosage is approximately 5 mg/kg which is (again approximately) about 2.3 mg/lb. Another way to look at this would be to double your dog's weight (2 mg/lb). Example: if your dog weighs 30 lbs, the initial recommended daily dosage could be 2.3 mg PB x 30 lbs (the weight of your dog) = 69 mg/day. Or simply doubling your dogs weight (30 lbs in this example) would be 60 mg/day. Therefore you could give your dog either a 30 mg tablet every 12 hours or split a 60 mg (grain) tablet and give a 1/2 grain every 12 hours.

As mentioned above in addition to considering the weight of your dog, the severity of the seizures is also taken into consideration. For example, for a dog with fairly severe or frequent seizures, your veterinarian may use a relatively higher starting dose. For a dog with infrequent, mild seizures, your veterinarian may often use a lower dose (to minimize side effects) and gradually increase the dose if necessary,

The first PB level should be taken 2 weeks after starting PB therapy. Based on the 'trough' levels and on how well the seizures are controlled, your veterinarian might leave the initial dosage the same or adjust it. Ideally trough levels should be taken 14, 45, 90 & and 180 days after starting PB therapy and at least every 6 months thereafter.

What is a loading dose?
Because it can take up to 7-14 days for PB to get into the system to become effective (reach steady-state serum levels), some veterinarians might prescribe a loading dose during this period. Where the normal starting dosage is 5 mg/kg/day the loading dosage can be anywhere between 5 and 16 mg/kg/day.

How do I know that my dog is receiving the proper doseage?
Each dog is different, so is their ability to metabolize medications. The only way to determine if your dog is getting the proper dosage is to monitor the PB serum concentrations (levels). This is done with a blood test. The first level is usually taken 2 weeks after initially starting PB therapy.

Based on the level results and also on how well the seizures are controlled, the dosage can be adjusted. By the way, if the PB blood serum level results come back and are not what you expected and there is no known reason for them to be 'off', ask to have the test re-run. Levels should be taken at least twice a year, if not more often.

Levels...what do the numbers mean?
When the lab sends your veterinarian the results from a PB serum level test, they will also note the range that the lab uses. One suggested range is 20-35 ug/ml (86-150umol/l in countries that use micromoles), another is 15-45 ug/ml and yet another is 15-40 ug/ml. In general the low level of the therapeutic range means that 50% of the dogs at the low level will exhibit some benefit (ie: seizure control) from the PB. Likewise, 50% of the dogs that are at the upper limit will NOT show signs of PB toxicity. As you can see, these lower and upper therapeutic ranges are a little vague. Therefore it is just as important to take into consideration your dog's seizure control and side effects rather than just relying on the numbers.

Trough and peak levels.
In the beginning it is important to test the trough level. A trough level represents the PB serum concentration when it is at it's lowest in the dog's system. If this level is too low, your dog could have a seizure. Therefore it is important in the beginning that this level is within therapeutic range. Some suggest that a minimum trough level is 20.

After your dog has been on PB and steady-state levels have been reached, a PB peak level can be taken. While the trough level is taken just before the next dosage is due, the peak level is taken anywhere from 4-6 hours (some say 4-8 hours) after PB dosage. Also if your dog has been given food, the food can delay the 'peak' period by several hours. So as you can see, determining the 'peak' time is not exactly precise. The peak level represents the maximum amount of PB in the system and is important for determining whether your dog might be reaching toxic levels. There are other signs of toxicity too - ataxia and the beginning of liver disease being two of them.

The seizures are now well controlled, can I reduce the dosage?
This should ONLY be done under the direct supervision of your veterinarian. You should NEVER do this on your own without consulting with your veterinarian. If your dog is under the care of several veterinarians (say a 'traditional' practitioner and an 'alternative' practitioner) it is necessary that each of them are aware of what the other is recommending. This is very important. One of the causes of status (status epilepticus) is reducing the PB - so reduction should be done very carefully and with the knowledge and approval of your veterinarians.

If you've not added another AED, you should wait until your dog is seizure free from between 6 months and a year. Then you could gradually begin to reduce the PB dosage by maybe 30%. Withdrawing the PB too quickly can bring on a seizure - even status. How long you wait until reducing it again varies, but don't be in too much of a hurry to reduce it if there is not a reason to do so (like liver disease). If you've added another AED (like KBr) and the new AED has had time to reach steady-state levels (around a month), you can consider slowly reducing the PB. The PB can be reduced a little faster under this scenario than if it was the only AED that your dog was on.

Is there anything I can do if I think my dog is going to have a seizure?
Yes, if you notice that your dog is exhibiting a behavior that results in a seizure, you can increase the PB dosage for a short period of time until you are sure that the potential for having a seizure is over. Then you can return to the normal dosage. If you think a seizure might be brought on by stress (like company, visiting the vet or dog groomer), you could try using Rescue Remedy. The one thing that you NEVER want to do is give your dog acepromazine. Acepromazine can reduce the seizure threshold and thus bring on a seizure. Be sure to discuss this issue with your veterinarian.

What if I miss a doseage or am late giving it?
Give the dosage as soon as you realize that it was missed. Even if it is time to give the next dosage, give the missed dosage. If your dog is vomiting and can't keep the medication down, check with your veterinarian. Also see the above paragraph "IS THERE ANYTHING TO DO IF I THINK MY DOG MIGHT HAVE A SEIZURE?"

I'm not going to be home when the next doseage is due, what can I do?
This is tough. If you can, give the dosage earlier rather than later. If it is easier to give the dosage that day every 8 hours instead of every 12, divide the dosage into thirds and give. And if you give the dosage really early, you might want to give an extra dosage when you get home. If you have a responsible neighbor or friend that could stop in and give your dog the medication, that would be perfect. Another thing you could try (but I would try this out before you needed to use it) would be to get one of the automatic cat feeders. You could set the timer to release the food (along with the medication) at a specified time. Just make sure that your dog will take the pills this way and not just eat the food - leaving the pills behind.

What drugs should not be taken whilst my dog is on PB?
Acepromazine is a definite no for dogs that have seizures. Remind your veterinarian that your dog is on PB when another drug is prescribed - this is important for over-the-counter medications too. There is a wonderful paperback book "The Pill Book Guide to Medication For Your Dog and Cat", Bantam Books. By all means look up the prescribed or over-the-counter drug in this book before giving it to your dog. Don't be afraid to question any medication that your dog is given.

When seeing another vet for the first time and you complete the health history on your dog, under the section 'is your dog allergic to anything', make sure that you write down acepromazine and also note that your dog is on phenobarbital.

Not meaning to get too technical here, but PB is a highly protein bound drug. This protein is not to be confused with the protein in your dog's food. If two highly protein bound drugs are given together, they will compete for protein carriers and may, depending on the drugs, interfere with each others absorption and distribution through the body/bloodstream. For example, Rimadyl and the barbiturates may cause one or both of the drugs to be less effective due to competition for protein binding sites, therefore dosages may have to be adjusted to either control the seizures or control the arthritis better.

What if my dog and I are out of town and I forgot the medication?
Please, please, please double check before you leave home that you have enough PB (and any other medication) with you for your dog. If you leave home without money, you can get some from an ATM machine. If you leave home without your dog's PB, it could be very hard to obtain - resulting with your dog having seizures. If going to a vet that is not your own (one that you have not established a client/patient relationship with) it could be VERY VERY difficult to obtain PB or for that matter any other medication that your dog needs. Always carry the phone number for your vet with you, preferably both the clinic and home phone number. Also keep a list with you of all the medication (name, dosage, brand) that your dog is on.

What if my dog's seizures are not controlled by PB?
The most important thing when giving medication (especially an AED) is compliance. If your dog is on PB twice a day, it needs to be given approximately every 12 hours - every day. So ask yourself, "have I been faithful in giving my dog his/her medication on time & every day?" If the answer is no, then this could be possibly the problem. Then ask yourself "is my dog on a different type of food, a different medication, lost weight,gained weight, has anything been changed from when the seizures were last under control?". If so, discuss this with your veterinarian. If the seizures are occurring around the time that the next dosage is to be given, you should look at the trough values. If this is the case, you could also look into giving the PB 3 times a day instead of twice. If the time between seizures is satisfactory, the peak levels within therapeutic range, but your dog is experiencing cluster seizures (multiple seizures in a 24 hour period), you could look into adding an additional AED - namely Potassium Bromide (KBr). If your dog's seizures are refractory (difficult to control) with just PB, adding KBr could make a great difference. One study reported that dogs that were refractory to PB, 86% of them had improved seizure control and 26% of the 86% became totally seizure free when adding KBr.

I'd like to read more about PB any recommendations?
Yes, see the article "Seizures in Dogs" by Dr Michael Podell, MSc, DVM that appeared in the July 1996 issue of Veterinary Clinics of North America: Small Animal Practice, Vol 26, Number 4 (pages 779-809).

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The above information is NOT intended to replace that provided by your veterinarian. Always discuss any changes regarding your dog's health or medication with your veterinarian.

 

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Page last update: 12/13/2011

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