By Rania Gollakner, BS, DVM, MPH

What is phenobarbital?

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

Its use in 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 direction may be significantly different from those on the label.

How is phenobarbital given?

Phenobarbital is given by mouth in the form of a tablet, capsule, liquid solution, paste, or chewable. 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. Phenobarbital must be given regularly, do not skip doses. Do not stop this medication abruptly, this medication must be tapered slowly.

At the clinic or hospital, it can also be given as an injection.

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, but if it is close to the time for the next dose, skip the dose you missed and 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, agitation, lethargy, or sedation when therapy is started, and increased thirst, urination, and appetite or elevated liver enzymes on bloodwork are possible. Side effects such as incoordination may indicate a dose that is too high. Less commonly, decreased blood cell counts may occur. Although rare, if signs of liver dysfunction are observed (yellowing of the skin, vomiting, anorexia, abnormal bloodwork, skin ulcers) or liver enzymes are 4-5 times higher than normal on bloodwork, a different medication should be started immediately and phenobarbital discontinued.

Cats can exhibit incoordination, sedation, lethargy, facial itchiness, increased appetite and associated weight gain, and increased thirst and urination. Rarely, cats can have decreased blood cell counts on bloodwork.

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

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

Are there any risk factors for this medication?

Do not use phenobarbital in pets that are allergic to it or that have kidney inflammation/infection, severe breathing problems, or severe liver disease. It should be used cautiously in pets that are anemic, dehydrated, have poor adrenal function, or heart/lung disease and in pets that are pregnant or lactating.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with phenobarbital: acetaminophen, benzodiazepines, carprofen, monoamine oxidase inhibitors, opioids, phenytoin, and rifampin.

Some drugs may increase the effects of phenobarbital, including antihistamines, bromides, chloramphenicol, felbamate, fluconazole, opioids, phenothiazines, urinary acidifiers, and valproic acid.

Phenobarbital can also decrease the effects of certain drugs such as anticoagulants, beta-blockers, bupropion, chloramphenicol, clarithromycin, clonazepam, corticosteroids, cyclosporine, dexamethasone, doxorubicin, doxycycline, estrogens, felbamate, griseofulvin, itraconazole, ketoconazole, lamotrigine, levetiracetam, levothyroxine, medroxyprogesterone, methadone, methylprednisolone, metoprolol, metronidazole, quinidine, paroxetine, phenothiazines, praziquantel, prednisone, progestins, tacrolimus, theophylline, topiramate, tricyclic antidepressants, urinary alkalinizers, valproic acid, verapamil, vincristine, voriconazole, warfarin, and zonisamide.

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

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, and watch closely for side effects. Phenobarbital levels will also be monitored routinely to confirm proper medication dosing.

How do I store phenobarbital?

Tablets and liquid medication should be stored at room temperature between 15°C to 30°C (59°F to 86°F) and protected from light and moisture.

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.