By Lauren Forsythe, PharmD, DICVP; Rania Gollakner, BS, DVM, MPH

What is phenobarbital?

Phenobarbital, also known as phenobarbitone, fenobarbital, phenemalum, phenobarbitalum, phenylethylbarbituric acid, and phenylethylmalonylurea (brand names: Nobatol ®, Luminal Sodium®, Solfoton®, Tedral®) is a barbiturate used to treat seizures and as a sedative in cats, dogs, ferrets, and horses.

Phenobarbital is used in dogs to treat seizures due to generalized epilepsy, and its use in other small and large animals to treat seizures or as a sedative is “off label” or “extra label”. Many drugs are commonly prescribed for off-label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully, as their directions may be significantly different from those on the label.

How is phenobarbital given?

  • Give phenobarbital is given by mouth in the form of a tablet, capsule, liquid syrup, paste, or chewable.
  • It can also be given as an injection at the clinic or hospital.
  • It may be given with or without food. 
  • If vomiting occurs when dosed on an empty stomach, give future doses with food or a treat. 
  • This medication may also be compounded into a transdermal form to apply topically to a cat’s ear.
  • Phenobarbital must be given regularly; do not skip doses. 
  • Do not stop this medication abruptly; it must be tapered slowly. 

This medication can take a few weeks before full effects are noted, but gradual improvements are usually noticeable after several doses.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember. However, if it is close to the time for the next dose, skip the dose you missed, give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

Common side effects in dogs include:

  • anxiety or agitation
  • lethargy 
  • transient sedation when therapy is started or dose is increased

Possible side effects in dogs include:

  • increased thirst, urination, and appetite 
  • elevated liver enzymes on bloodwork
  • incoordination (may indicate a dose that is too high)
  • decreased blood cell counts (less common)

Immediately STOP giving phenobarbital and use a different medication if you see signs of liver dysfunction, including:

  • yellowing of the skin
  • vomiting oranorexia
  • abnormal bloodwork
  • skin ulcers
  • liver enzymes that are 4–5 times higher than normal on bloodwork

Common side effects in cats include:

  • incoordination
  • sedation or lethargy
  • facial itchiness
  • increased appetite and associated weight gain
  • increased thirst and urination

Rarely, cats can have decreased blood cell counts on bloodwork.

Phenobarbital can alter the results of some thyroid and Cushing’s testing.

This moderate-acting medication should stop working in a few days, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Do NOT use phenobarbital in pets that:

  • are allergic to it
  • have kidney inflammation/infection
  • have severe breathing problems
  • have severe liver disease

Use phenobarbital cautiously in pets that:

  • are anemic
  • are dehydrated
  • have poor adrenal function
  • have heart/lung disease
  • are pregnant or lactating

Are there any drug interactions I should be aware of?

Certain medications should be used with caution when given with phenobarbital, including but not limited to:

  • acetaminophen
  • benzodiazepines
  • carprofen
  • opioids

Some drugs may increase the effects of phenobarbital, including but not limited to:

  • antihistamines
  • bromides
  • chloramphenicol
  • fluconazole
  • opioids

Phenobarbital can also decrease the effects of certain drugs such as:

  • anticoagulants 
  • atenolol
  • certain anti-fungal medications, including itraconazole and ketoconazole
  • certain anti-seizure medications, including levetiracetam and zonisamide
  • chloramphenicol
  • clonazepam
  • corticosteroids, including dexamethasone and prednisone
  • cyclosporine
  • doxycycline 
  • levetiracetam
  • levothyroxine
  • methadone 
  • metronidazole 
  • praziquantel
  • theophylline

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any new treatment.

Is there any monitoring that needs to be done with this medication?

  • You and your veterinarian should monitor your pet to be sure that the medication is working.
  • Track the occurrence of seizures to help monitor how well this medication is working.
  • Monitor your pet for any adverse effects.
  • Your veterinarian will monitor phenobarbital levels routinely to confirm proper medication dosing.

How do I store phenobarbital?

  • Store tablets and liquid medication at room temperature between 15°C and 30°C (59°F to 86°F).
  • Store protected from light and moisture.
  • Store in a locked cabinet or safe and monitor the amount of medication remaining.

Phenobarbital is a controlled substance, which means that it has been designated by the Drug Enforcement Administration (DEA) as having a potential for diversion to people other than the patient it was prescribed for. Use of a controlled substance in any person or animal other than that for which it is prescribed is illegal. Therefore, this medication should be stored in a locked cabinet or safe that cannot be easily moved, with access limited to only those that need to administer the medication. Monitor the amount of medication remaining to ensure the expected amount is present.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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