This article describes possible side effects of phenobarbital oral tablet. You can explore this overview article to learn more about phenobarbital, including the drug’s uses, ways to save on cost, and more.
Phenobarbital oral tablet is a generic medication. It isn’t available in a brand-name version. Phenobarbital also comes in an injectable form and an oral liquid, but these forms aren’t described in this article. To learn more about these forms, talk with your doctor.
Note: The Food and Drug Administration (FDA) has not approved phenobarbital oral tablet. 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 a boxed warning about the risks of serious harm if taken with opioids. It also has a boxed warning about the risks of misuse and addiction, dependence, and withdrawal.* Boxed warnings are the most serious warnings about the safety of certain medications.
For details on the risk of serious effects if taken with opioids, see the “Side effects of phenobarbital in detail” section below. For more information about the other risks, see the “Withdrawal, dependence, misuse, and phenobarbital” section below.
* These warnings were actually issued for the brand name Sezaby injectable form of the drug, but this form contains the same active ingredient as phenobarbital oral tablet.
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More common side effects of phenobarbital
Phenobarbital oral tablet may cause mild or serious side effects. Below are some of the more common side effects of this medication.
Commonly reported side effects of phenobarbital oral tablet include:
- sleepiness
- lethargy
- vertigo
- nausea and vomiting
- headache
Mild side effects of phenobarbital
Phenobarbital may cause certain mild side effects. Usually, mild side effects are temporary and can be easily managed. Tell your doctor or pharmacist if you have side effects that don’t go away or become bothersome.
Mild side effects reported with phenobarbital oral tablet include:
- sleepiness
- lethargy
- vertigo
- nausea and vomiting
- headache
- confusion
- agitation
- mild allergic reaction*
Phenobarbital oral tablet may cause more mild side effects than those listed above. For details, see the drug’s prescribing information.
* To learn more about this side effect, see the “Side effects of phenobarbital in detail” section below.
Serious side effects of phenobarbital
Phenobarbital may cause certain serious side effects. If you have any serious side effects from phenobarbital oral tablet, call your doctor right away. But if you think you’re having a medical emergency or your side effects feel life threatening, take immediate action. Call 911 or a local emergency number, or go to the nearest emergency room.
Serious side effects reported with phenobarbital oral tablet include:
- residual sedation (a “hangover” effect)
- respiratory depression*
- slow heart rate
- low blood pressure
- severe skin reactions, such as Stevens-Johnson syndrome
- risk of serious effects if taken with opioids*†
- risk of withdrawal and dependence†‡
- risk of misuse and addiction†‡
- severe allergic reaction*
* To learn more about this side effect, see the “Side effects of phenobarbital in detail” section below.
† Phenobarbital has a boxed warning for this side effect. Boxed warnings are the most serious warnings about the safety of certain medications.
‡ To learn more about this side effect, see the “Withdrawal, dependence, misuse, and phenobarbital” section below.
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Get free cardSide effects of phenobarbital in children
For children, doctors prescribe phenobarbital oral tablet to manage seizures. The dosage depends on the child’s weight.
In studies, the following side effects were more common in children than in adults:
- memory problems
- excitement
- irritability
- hyperactivity
If you’d like to know more about side effects in children taking phenobarbital oral tablet, talk with your doctor or pharmacist.
Common questions about side effects of phenobarbital
Here are answers to some common questions about side effects of phenobarbital oral tablet. If you have other questions, talk with your doctor or pharmacist.
Can phenobarbital be used in dogs? How do side effects of the drug in dogs compare with those seen in humans?
Yes, phenobarbital can be prescribed to dogs. A phenobarbital oral tablet is approved by the FDA to manage certain types of seizures in dogs.
The side effects of phenobarbital oral tablet in dogs may be different than in humans. Talk with your dog’s veterinarian to learn more about the use of phenobarbital in dogs and its possible side effects.
Do not give phenobarbital oral tablet or any other medications to your pet without talking with their veterinarian first.
Is phenobarbital safe for older adults?
Not necessarily. The prescribing information for phenobarbital oral tablet notes that older adults (ages 65 years and older) may be more sensitive to the effects of barbiturates. Phenobarbital is a barbiturate. Doctors recommend a lower dose of phenobarbital for older adults.
Older adults may also be more likely to experience confusion, slowed brain activity, or nervousness while taking phenobarbital oral tablet.
Guidelines from the American Geriatrics Society recommend avoiding barbiturates, including phenobarbital, in older adults. This is because of the risk of dependence and a higher risk of overdose, even with low doses of phenobarbital.
Talk with your doctor or pharmacist to learn more about how your age could affect your risk of side effects with phenobarbital.
Does phenobarbital cause long-term side effects?
Yes, in some cases, phenobarbital oral tablet may cause long-term side effects. These can occur if you take the drug for a long time or may last for a while after you’ve stopped taking the drug.
Examples of long-term side effects reported with phenobarbital oral tablet include:
- misuse, dependence, and withdrawal*
- a blood disorder known as megaloblastic anemia
Talk with your doctor to learn more about these side effects and how to manage them. Ask your doctor for more information about how long these side effects of phenobarbital may last.
Also talk with your doctor if you have more questions about possible long-term side effects with phenobarbital.
* To learn more about these side effects, see the “Withdrawal, dependence, misuse, and phenobarbital” section below.
Withdrawal, dependence, misuse, and phenobarbital
Phenobarbital oral tablet has a boxed warning about the risks of misuse and addiction, dependence, and withdrawal.* Boxed warnings are the most serious warnings about the safety of certain medications.
Here are some definitions for these terms:
- with misuse, a drug is taken in a way other than how it’s prescribed
- with dependence, your body needs the drug to function like usual
- withdrawal describes symptoms that can occur when you stop taking a drug your body is dependent on
Phenobarbital oral tablet is a controlled substance. A controlled substance is a drug the government regulates due to the risk of dependence or misuse. Misuse of phenobarbital can lead to overdose and death.
You should take phenobarbital exactly as your doctor prescribes. Withdrawal symptoms from stopping phenobarbital abruptly can be severe and life threatening. They may include:
Certain factors can increase the risk of misuse, dependence, and withdrawal with phenobarbital oral tablet. These include a prior history of misusing medications or taking high doses of phenobarbital for a long period of time.
Due to these risks, your doctor will likely prescribe the smallest amount needed to last you until your next appointment and the lowest dose required to treat your condition.
Do not stop phenobarbital abruptly without talking with your doctor. If you and your doctor decide that you should no longer take phenobarbital, your doctor will lower the dose slowly over time, especially if you’ve taken it for a long time. This can help reduce the risk of withdrawal.
* This warning was actually issued for the brand name Sezaby injectable form of the drug, but this form contains the same active ingredient as phenobarbital oral tablet.
Side effects of phenobarbital in detail
Below you’ll find details on some of the side effects of phenobarbital oral tablet.
Risk of serious effects if taken with opioids
Phenobarbital has a boxed warning about the risks of serious side effects if taken with opioids.* Boxed warnings are the most serious warnings about the safety of certain medications.
Taking phenobarbital oral tablet with an opioid may increase the risk of the following side effects:
- extreme sleepiness
- difficulty breathing
- coma
- death
Both phenobarbital and opioid drugs can cause central nervous system (CNS) depression. Your CNS consists of your brain and spinal cord. It coordinates activities throughout your body, including breathing.
With CNS depression, the activities controlled by the CNS slow down. Severe CNS depression can occur if phenobarbital is taken with an opioid. This can lead to serious or life threatening side effects, including difficulty breathing, extreme sleepiness, coma, and death.
Certain factors can further increase the risk of harm if you take phenobarbital with an opioid. These factors include taking these drugs with alcohol or other CNS depressants. Your risk may also go up with higher doses of phenobarbital or opioids or when either drug is taken for a long period of time.
Note: Phenobarbital also has a boxed warning about the risk of misuse and addiction, dependence, and withdrawal. To learn more, see the “Withdrawal, dependence, misuse, and Phenobarbital” section above.
* This warning was actually issued for the brand name Sezaby injectable form of the drug, but it contains the same active ingredient as phenobarbital oral tablet.
Ways to manage
If you have concerns about the risks of harm with opioids and phenobarbital, tell your doctor. Due to these risks, your doctor will likely not prescribe phenobarbital along with an opioid, unless there are no other treatment options.
If both medications are needed, your doctor will prescribe the lowest possible dose of each drug. They’ll also recommend that you take these drugs for the shortest duration needed to treat your conditions.
Your doctor will also monitor you for sleepiness, difficulty breathing, and other signs and symptoms of CNS depression. You should also avoid driving or operating heavy machinery until you know how this combination affects you.
You or someone else should call 911 or go to the closest emergency room if you have symptoms that feel life threatening, such as excessive sleepiness or slow, shallow breathing.
If you must take an opioid together with phenobarbital, your doctor or pharmacist may also recommend having naloxone on hand. Naloxone can help treat severe breathing problems that may occur if phenobarbital and an opioid are taken together.
Respiratory depression
Respiratory depression was reported in studies of people taking phenobarbital oral tablet. This side effect may be serious.
With respiratory depression, you may have slow, shallow breathing. Symptoms may include:
- shortness of breath
- sleepiness
- confusion
- seizures
- blue-colored lips, fingers, or toes
Certain factors can increase the risk of respiratory depression with phenobarbital oral tablet. These include:
- taking higher doses
- overdose
- drinking alcohol
- taking an opioid* or other drugs that can cause respiratory depression
- lung problems
- certain other medical conditions
* For details on the risk of serious effects if phenobarbital is taken with opioids, see the “Side effects of phenobarbital in detail” section above.
Ways to manage
If you have concerns about respiratory depression, tell your doctor. They can discuss the risks and benefits of taking phenobarbital oral tablet. They can also help you decide whether phenobarbital is the best option for your condition. And if you take phenobarbital, they can give you advice about how to lower your risk of respiratory depression.
Due to this risk, your doctor will likely prescribe the lowest possible dosage of phenobarbital oral tablet for your condition. They’ll also prescribe phenobarbital for the shortest duration needed.
You or another person should call 911 right away if you:
- have slow, shallow breathing, or if your breathing stops completely
- become very sleepy or can’t be woken up
- become unresponsive
These symptoms require immediate medical care because they can be life threatening.
Allergic reaction
For some people, phenobarbital oral tablet can cause an allergic reaction.
Symptoms can be mild or serious, and they may include:
- itchiness
- skin rash
- swelling under the skin, usually affecting eyelids, lips, hands, or feet
- swelling of the mouth, tongue, or throat, which can make it hard to breathe
- flushing (temporary warmth, redness, or deepening of skin color)
Ways to manage
For mild allergic reaction symptoms, such as a mild rash, call your doctor right away. They may recommend treatments to help manage your symptoms. They’ll also let you know whether you should keep taking the medication.
For severe allergic reaction symptoms, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms require immediate medical care because they can become life threatening. If you’ve had a serious allergic reaction to phenobarbital oral tablet, your doctor may recommend taking a different medication instead.
Warnings for phenobarbital
Below are some factors you may want to discuss with your doctor before taking phenobarbital oral tablet.
Boxed warnings
Phenobarbital has a boxed warning about the risks of serious harm if taken with opioids. It also has a boxed warning about the risks of misuse and addiction, dependence, and withdrawal.* Boxed warnings are the most serious warnings about the safety of certain medications.
For details on risk of serious effects if taken with opioids, see the “Side effects of phenobarbital in detail” section above. For more information about the other risks, see the “Withdrawal, dependence, misuse, and phenobarbital” section above.
* These warnings were actually issued for the brand name Sezaby injectable form of the drug, but this form contains the same active ingredient as phenobarbital oral tablet.
Other warnings
It may not be safe for you to take phenobarbital oral tablet if you have specific health conditions. These are sometimes called drug-condition interactions. Other things may also affect whether phenobarbital oral tablet is a safe treatment option for you.
Before taking phenobarbital oral tablet, tell your doctor about your health and the medications you take. Things to consider include:
- kidney impairment
- liver disease
- depression
- history of drug or alcohol misuse
- phobias
- mental health conditions
- acute or chronic pain
- asthma, hives, or allergic conditions
- past allergic reaction to phenobarbital
- pregnancy and breastfeeding
Phenobarbital and pregnancy
Phenobarbital oral tablet should not be taken during pregnancy unless the benefits outweigh the risks. Barbiturates, like phenobarbital, can cause harm to the fetus if taken during pregnancy.
If you’re planning a pregnancy or can become pregnant, talk with your doctor before taking this medication. Your doctor may suggest birth control options to use during treatment with phenobarbital. If you’re already taking phenobarbital, don’t stop it without talking with your doctor first.
Phenobarbital and breastfeeding
Phenobarbital oral tablet should not be taken while breastfeeding. Small amounts of barbiturates, such as phenobarbital, pass into breast milk.
If you’re breastfeeding or planning to do so, talk with your doctor before taking this medication. Your doctor may recommend other ways to feed your child during treatment with phenobarbital.
Phenobarbital and alcohol
Phenobarbital oral tablet is known to interact with alcohol. The prescribing information states that you should not consume alcohol while taking a barbiturate, such as phenobarbital.
Consuming alcohol with phenobarbital oral tablet can increase the risk of respiratory depression* or other symptoms of CNS depression.
Severe CNS depression can lead to serious or life threatening side effects, including difficulty breathing, extreme sleepiness, coma, and death. Taking multiple CNS depressants together can increase these risks. Both alcohol and phenobarbital are CNS depressants.
If you have a history of alcohol use disorder, you may also be more likely to misuse phenobarbital or develop dependence on the drug. With misuse, a drug is taken in a way other than how it’s prescribed. With dependence, your body needs the drug in order to function like usual.
If you have questions about the risks of drinking alcohol during treatment with phenobarbital oral tablet, talk with your doctor or pharmacist.
* To learn more about this side effect, see the “Side effects of phenobarbital in detail” section above.
What to ask your doctor
This article describes common and serious side effects of phenobarbital oral tablet. If your doctor prescribes this medication, they can discuss possible side effects with you. Let your doctor know if you have concerns or questions about treatment with this medication.
Here’s a list of possible questions you may want to ask your doctor:
- Does my risk of side effects with phenobarbital oral tablet go up the longer I take it?
- Will I have a higher risk of side effects if you prescribe a higher dose of phenobarbital?
- Do the other medications I take interact with phenobarbital?
- How do the side effects of phenobarbital oral tablet compare with those of other drugs that could treat my condition?
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.