Phenobarbital

Written by Helen Marshall, BPharm, MRPharmS
Medically reviewed by Rosanna Sutherby, PharmD
Updated on June 20, 2023

This article describes phenobarbital’s uses, side effects, and more. You’ll also find some coupon options to help save on the cost of phenobarbital.

Note: The Food and Drug Administration (FDA) has not approved phenobarbital. This is because the
drug has been available since before the FDA began its drug approval process. If you have questions
about this, talk with your doctor or pharmacist.

Boxed warnings

Phenobarbital has boxed warnings about risks of taking the drug with opioids, and risks of misuse and addiction, dependence, and withdrawal. Boxed warnings are the most serious warnings about the safety of certain medications. For details, see the “Interactions and warnings for phenobarbital” section.

Below you’ll find answers to a few commonly asked questions about phenobarbital.

Is phenobarbital used for dogs or cats?

Yes, phenobarbital is sometimes prescribed for dogs and cats. The medication isn’t approved for use in pets, but veterinarians sometimes use it to treat seizures in dogs and cats. To learn more, talk with your veterinarian.

Do not give phenobarbital or any other medications to your pet without talking with your veterinarian.

Can phenobarbital be used for migraines or anxiety?

This isn’t likely. Phenobarbital is a sedative (a drug used to help you relax) that can be taken before certain medical procedures. It may be used to help reduce short-term anxiety and muscle tension in these situations. But doctors don’t often prescribe it specifically to treat anxiety or anxiety disorders. There are other anxiety drugs available that are considered safer options.

Phenobarbital is also unlikely to be prescribed for migraine treatment. However, butalbital, which is a sedative similar to phenobarbital, is sometimes used to relieve migraine symptoms.

Your doctor or pharmacist can provide details about treatment options for migraine or anxiety.

How does phenobarbital work? And how long does it take to start working?

Phenobarbital is thought to work by slowing activity in your brain and nerves.

When taken as a sedative (to help you relax) before medical procedures, phenobarbital causes sleepiness, relaxes muscles, and helps reduce anxiety and tension. When taken for seizures, phenobarbital works to stop surges of electrical activity that cause seizures from building up and spreading in your brain.

Phenobarbital typically starts working in about 5–15 minutes when it’s given by intravenous (IV) infusion (an injection given into a vein over time). When taken by mouth, it usually starts working in about 1 hour.

If you’d like to know more about this, talk with your doctor or pharmacist.

Uses for phenobarbital are described below. These uses may differ based on the form of phenobarbital prescribed.

Note: The Food and Drug Administration (FDA) has not approved phenobarbital. This is because the
drug has been available since before the FDA began its drug approval process. If you have questions
about this, talk with your doctor or pharmacist.

Use for seizures

Doctors may prescribe phenobarbital for treating seizures. It’s used for this purpose in adults and children. It may be prescribed as a long-term or short-term treatment.

Phenobarbital can be used to treat seizures of all types, including focal onset seizures (also called partial seizures) and generalized tonic-clonic seizures. Seizures may occur in people with epilepsy or other seizure disorders. Or they may occur due to brain injury or other medical conditions.

Your doctor may prescribe phenobarbital as a long-term treatment to help prevent seizures.

In certain situations, doctors may give phenobarbital by injection or intravenous (IV) infusion (an injection given into a vein over time). This may be done to treat a seizure that’s very severe or doesn’t stop with other treatments.

Use for sedation

Doctors may prescribe phenobarbital for sedation (to help you relax) in certain situations. It’s used for this purpose in adults and children. It’s usually prescribed as a short-term treatment.

When used as a sedative, phenobarbital helps reduce anxiety and tension. It can make you feel calm, physically and mentally relaxed, and sleepy. It may also reduce awareness of your surroundings.

Your doctor may prescribe phenobarbital as a sedative before certain medical procedures or minor surgeries. You may also be given phenobarbital before certain surgeries that you’ll receive general anesthesia for.

Doctors may also prescribe phenobarbital as a sedative to reduce anxiety or help you sleep if you have certain medical conditions.

Below, you’ll find information about phenobarbital’s possible interactions and warnings.

Interactions of phenobarbital

Certain things may affect how a drug works. These include getting vaccines, consuming alcohol* or certain foods, or taking the drug with other medications. This effect is called a drug interaction.

Before you take phenobarbital, ask your doctor to check for possible interactions. Be sure to tell them about any of the following you take or use:

• prescription or over-the-counter drugs
• vitamins, herbs, or supplements

To learn about drug-condition interactions, see the “Warnings for phenobarbital” section below.

* You should not drink alcohol while taking phenobarbital. Talk with your doctor if you have concerns about avoiding alcohol during phenobarbital treatment.

Warnings for phenobarbital

Some people should not take phenobarbital and others should take it cautiously.

Boxed warnings

Phenobarbital has boxed warnings about the risks of use with opioids, misuse and addiction, dependence, and withdrawal. Boxed warnings are the most serious warnings about the safety of certain medications.

To learn more about these warnings, read below. You can also talk with your doctor or pharmacist.

Risks if taken with opioids. Taking phenobarbital with opioids may cause extreme sleepiness, breathing problems, coma, and even death. Due to these risks, doctors usually will not prescribe phenobarbital with an opioid. Opioids include strong pain relievers such as oxycodone (Oxycontin).

If you need to take phenobarbital with an opioid, your doctor will prescribe the lowest possible dosage of both medications. While taking these medications together, you or another person should call 911 right away if you:

• become very sleepy or can’t be woken up
• become unresponsive
• have breathing that’s slow, shallow, or stops completely

Risk of misuse and addiction. Phenobarbital has a risk of misuse and addiction. Misuse means a drug is taken in a way other than how it’s prescribed. With addiction, you feel unable to stop taking a drug, even if it’s causing harm.

Misuse of phenobarbital can lead to addiction, overdose, and death. You should only take phenobarbital as prescribed by your doctor.

Risk of dependence and withdrawal. If you take phenobarbital for a long time, you may become dependent on the drug. Dependence means the body needs the drug to function like usual.

Withdrawal symptoms can occur when you stop taking a drug your body is dependent on. Withdrawal symptoms can occur if you suddenly stop phenobarbital after taking the drug long term. These symptoms can be serious and sometimes life threatening. They may include anxiety, shaking, trouble sleeping, dizziness, delirium, and seizures.

You should not stop taking phenobarbital without talking with your doctor. If you’ve been taking phenobarbital for a long time, your doctor will likely reduce your dosage slowly before you stop treatment completely. This helps you avoid withdrawal symptoms.

Other warnings

Phenobarbital could cause harm to people with certain health conditions. This effect is called a drug-condition interaction. Other factors can also affect whether phenobarbital is a safe option for you. These warnings may differ based on the form of phenobarbital you take.

Ask your doctor about specific warnings for phenobarbital, and be sure to tell your doctor about your:

• current health, including any allergies to medications
• past health conditions or surgeries

Allergic reaction

Phenobarbital can cause an allergic reaction in some people. Severe allergic reactions are rare but possible.

If you’ve had an allergic reaction to phenobarbital or any of its ingredients, your doctor will likely not prescribe phenobarbital. They can tell you about medications that are safer options for you.

A mild allergic reaction may cause the following symptoms:

• skin rash
• itchiness
• flushing (temporary warmth, redness, or deepening of skin color)

A severe allergic reaction may cause the following symptoms:

• swelling under your skin, usually in your hands, feet, eyelids, or lips
• swelling of your mouth, throat, or tongue, which can cause breathing problems

If you have an allergic reaction to phenobarbital, call your doctor right away. If you have severe symptoms, call 911 or a local emergency number.

Overdose of phenobarbital

If you take phenobarbital by mouth, you should not take more than your doctor prescribes. For some drugs, doing so may lead to serious side effects or overdose.

If you think you’ve taken too much phenobarbital, call your doctor or pharmacist right away. Or you could call 800-222-1222 to speak with someone at America’s Poison Centers. You can also use its online resource. If you have concerning symptoms, call 911 or a local emergency number immediately. You can also go to the closest emergency room.

Pregnancy or breastfeeding and phenobarbital

Information about phenobarbital, pregnancy, and breastfeeding is described below.

Phenobarbital and pregnancy

There are risks to taking phenobarbital during pregnancy. But there are also risks with having untreated seizures while pregnant. (Keep in mind that phenobarbital can be used for treating seizures.) There may also be times when the benefits of taking phenobarbital as a sedative during pregnancy outweigh its risks.

If you’re planning a pregnancy or can become pregnant, talk with your doctor about your treatment options during pregnancy. Together, you can discuss the risks and benefits of your treatment options and decide whether you should take phenobarbital during pregnancy.

Phenobarbital and breastfeeding

It’s not known whether phenobarbital should be taken while breastfeeding. If you’re currently breastfeeding or planning to do so, talk with your doctor before taking this medication.

Phenobarbital may cause mild or serious side effects. More common mild side effects of phenobarbital and its serious side effects are listed below. This article doesn’t include all possible side effects of the drug. Side effects can vary based on your age, overall health, and any other medications you take.

To learn more about phenobarbital’s side effects, ask your doctor or pharmacist. You can also read the prescribing information for phenobarbital in the “Article resources” section.

Mild side effects

More common mild side effects reported with some forms of phenobarbital are listed below. These side effects may differ based on the form of phenobarbital you take.

This article doesn’t include all possible mild side effects of the drug. To learn more about phenobarbital’s side effects based on the form, see the prescribing information for phenobarbital in the “Article resources” section.

With many drugs, mild side effects may go away within a few days or a couple of weeks. If the side effects are bothersome, tell your doctor or pharmacist. They may be able to suggest ways to manage them.

Mild side effects of phenobarbital include:

• sleepiness
• memory problems
• confusion
• unsteadiness
• mild allergic reaction*

* For details about this side effect, see the “Interactions and warnings for phenobarbital” section.

Serious side effects

Serious side effects of phenobarbital are listed below. These side effects may differ based on the form of phenobarbital you take.

This article doesn’t include all possible serious side effects of the drug. To learn more about phenobarbital’s side effects based on the form, see the prescribing information for phenobarbital in the “Article resources” section.

With many drugs, serious side effects are possible but not common. If you have serious side effects from this drug, call your doctor right away. If you’re having severe symptoms or a medical emergency, call 911 or a local emergency number.

In general, phenobarbital’s serious side effects include:

• slow, shallow breathing
• low blood pressure
• serious skin reactions
• boxed warnings: risks if taken with opioids, and risks of misuse and addiction, dependence, and withdrawal*
• severe allergic reaction*

With phenobarbital intravenous (IV) infusion,† the following serious side effects may also occur:

• tissue damage at the injection site

* For details about this side effect, see the “Interactions and warnings for phenobarbital” section.
† An IV infusion is an injection given into a vein over time.

Suicide prevention

If you or someone you know is in crisis and considering suicide or self-harm, please seek support:

• Call the 988 Suicide and Crisis Lifeline at 988.
• Text HOME to the Crisis Textline at 741741.
• Not in the United States? Find a helpline in your country with Befrienders Worldwide.
• Call 911 or your local emergency services number if you feel safe to do so.

If you’re calling on behalf of someone else, stay with them until help arrives. You may remove weapons or substances that can cause harm if you can do so safely.

If you are not in the same household, stay on the phone with them until help arrives.

This article describes phenobarbital’s uses and side effects, ways to save on cost, and more. Let your doctor know if you have questions about phenobarbital or would like more details about it.

Here’s a list of some possible questions you may want to ask your doctor:

• How long will I take phenobarbital for?
• Will I need any monitoring while taking phenobarbital?
• Does phenobarbital have long-term side effects?
• Can phenobarbital affect my birth control?
• What should I avoid while taking phenobarbital?

• Food and Drug Administration. (2023). Orange Book: Approved drug products with therapeutic equivalence evaluations. https://www.accessdata.fda.gov/scripts/cder/ob/index.cfm
• Phenobarbital – Drug summary. (n.d.). https://www.pdr.net/drug-summary/Phenobarbital-Elixir-phenobarbital-2669
• Phenobarbital- phenobarbital liquid. (2023). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/ac26cd10-8f66-4254-957c-87a909b576f8/spl-doc
• Phenobarbital sodium- phenobarbital sodium injection. (2023). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/7741e9ab-54ea-4c7e-9d64-daff922704ba/spl-doc
• Phenobarbital- phenobarbital tablet. (2023). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/97bab152-6c7b-4eb6-b8cc-9f64d2a5bdbe/spl-doc
• Sezaby (phenobarbital sodium) for injection, for intravenous use. (2022). https://www.accessdata.fda.gov/drugsatfda_docs/label/2022/215910s000lbl.pdf

Disclaimer: Optum Perks has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.