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Phenobarbital

Supportive Care

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What is phenobarbital?

Phenobarbital is used to prevent seizures. It is also used to sedate patients (help them sleep during procedures) and to relieve anxiety. Phenobarbital is available as a 4 mg/ml reddish-orange liquid, and as white tablets in 15-mg, 30-mg, 60-mg, and 100-mg strengths. It is also available as a clear liquid given by vein (IV).

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Possible side effects

  • Nausea and vomiting
  • Feeling dizzy, drowsy, and unsteady
  • Low blood pressure (IV)
  • Slower than normal rate of breathing (IV)

Symptoms of an allergic reaction include: rash, hives, itching, chills, fever, headache, muscle ache, shortness of breath, coughing, tightness in the throat, swelling of the face or neck.

Not all patients who take phenobarbital will experience these side effects. Common side effects are bolded, but there may be others. Please report all suspected side effects to your doctor or pharmacist.

Find more information on side effects.

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Possible late effects

  • Nausea, vomiting, or constipation
  • Blurred or double vision
  • Decreased coordination
  • Feeling dizzy, drowsy, and unsteady
  • Feeling tired
  • Agitated and irritable
  • Slurred speech
  • Headache
  • Feeling depressed
  • Excited
  • Trouble sleeping, nightmares
  • Low blood counts
  • Temporary changes in liver function
  • Allergic reaction with these symptoms: fever, rash, swelling of face and lips, and muscle aches

These are the most common side effects, but there may be others. Please report all side effects to the doctor or nurse.

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Tips for patients and families

Be sure to discuss all questions and instructions with your doctor or pharmacist. 

  • Blood levels of phenobarbital will be checked regularly to reduce side effects and to improve treatment. It is best to draw blood for these checks just before the next dose is due. Therefore, do not take your next scheduled dose until after the blood sample has been drawn. Your dose of phenobarbital may change based on the blood level.
  • If you have blurred vision, decreased coordination, vomiting, or an increased drowsy feeling, please call your doctor right away. These side effects can mean that the level of phenobarbital in your blood may be too high.

Drug at Home:

  • Do not stop taking phenobarbital until your doctor tells you to do so. If you stop taking it without being told, your chances of having a seizure may increase.
  • Phenobarbital may cause you to feel dizzy and drowsy. Do not operate heavy equipment or drive a motor vehicle until you see how this medicine will affect you.
  • Your doctor will slowly increase the dose of phenobarbital when you first begin taking it.
  • If you develop a rash after starting phenobarbital, it may mean you have an allergy to the medicine. Call your doctor, pharmacist, or nurse right away.
  • Store phenobarbital at room temperature.
  • It is very important that you take phenobarbital exactly as your doctor instructed. This will keep the blood levels constant during the day and night. If you miss a dose, follow these guidelines:
    • If the next dose is not due for at least 4 hours, take the missed dose right away.
    • If the next dose is due in less than 4 hours, do not take the missed dose. Take the next dose at the scheduled time. Never take a double dose unless the doctor or pharmacist tells you to do so.
  • Phenobarbital may affect the way several other medicines work. These medicines include warfarin, cyclosporine, theophylline, steroids, birth control pills, other seizure medicines, griseofulvin, and doxycycline.
  • Several medicines can affect blood levels of phenobarbital or increase the drowsy feeling and sedation caused by phenobarbital. These medicines include other seizure medicines, pain medicines, sedatives, and antihistamines (found in over-the counter sleeping pills and allergy medicines, such as Benadryl®). Alcohol (found in many over-the-counter cough/cold medicines) can also increase the drowsy feeling caused by phenobarbital.
  • Tell your doctor if you are taking any of these medicines or if you start taking any new medicine while taking phenobarbital.
  • The staff will draw blood samples regularly to check for changes in liver function.
  • This medicine can increase the risk of falls.
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Always check the medication label. Many medicines look the same. Do not take any new medicine, including over-the counter medicines or supplements, without checking with your doctor or pharmacist. Talk to your doctor or pharmacist about what to do in the case of a missed dose, allergic reaction, or medical emergency. 

This information is not intended to be a complete guide and is not a substitute for talking with your provider. Medicine names, appearance, formulas, or availability may vary by clinic location. Always follow the instructions given by your care team.

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