Phenobarbital dosage, forms, and strengths

Phenobarbital is a generic barbiturate widely used as an anticonvulsant and sedative. For seizure disorders, people take doses orally one to three times daily. One dose at bedtime is all that’s needed to treat insomnia. People who can’t take tablets can opt for a liquid formulation of phenobarbital. In certain situations, healthcare professionals will administer phenobarbital injections, either intravenous or intramuscular.

Phenobarbital forms and strengths

Phenobarbital is a widely used drug that comes in several oral dosage forms and as an injection for intramuscular or IV administration:

• Tablets: 15 mg, 16.2 mg, 30 mg, 32.4 mg, 60 mg, 64.8 mg, 97.2 mg, 100 mg

• Oral solution or elixir (liquid): 20 mg/5 mL

• Injection: 10 mg/mL, 65 mg/mL, 140 mg/mL

Phenobarbital dosage for adults

Phenobarbital is a sedative, hypnotic (sleep inducer), and anticonvulsant used for sedation or the treatment of partial and generalized seizures. Doctors use phenobarbital injections for daytime sedation, procedural sedation, insomnia, and long-term control of seizures. 

Phenobarbital is not safe for everyone. See the FAQ “Who should not take phenobarbital” below for a list of contraindications.

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Phenobarbital dosage for epilepsy

Phenobarbital is a first-line maintenance treatment for seizure disorders. Daily doses help prevent both partial and generalized seizures. 

As a barbiturate, phenobarbital controls seizures by slowing down nerve activity in general. Healthcare professionals often start seizure maintenance treatment with phenobarbital or another drug as the sole treatment. If seizures still occur, doctors often add another antiepileptic drug to the treatment regimen.

Doses are taken by mouth two or three times daily. 

• Standard dosage for the maintenance treatment of seizure disorders: 30–100 mg taken by mouth one to three times daily OR 60–200 mg daily

• Maximum dosage for the maintenance treatment of seizure disorders: 400 mg daily

Phenobarbital dosage for active seizures

Most seizures only last for a short time without any need for medication. When medications are used to stop an active seizure, the preferred treatments are sedatives called benzodiazepines, such as Valium (diazepam). Some can be administered by a caregiver as oral doses, rectal gel, or a nasal spray. 

In an emergency room or hospital, injections (intravenous or intramuscular), including phenobarbital injections, are also used to end convulsive seizures. 

Serious convulsive seizures (tonic-clonic seizures) are the most common type of generalized seizure, affecting about 1 out of 4 people with seizures. They usually pass in a few minutes. When drugs are required, healthcare professionals typically start with a benzodiazepine injection but can use phenobarbital injections.

The most serious type of seizure is status epilepticus, a seizure that lasts for five minutes or longer. It is a life-threatening emergency that requires immediate medical treatment to stop the seizure. The drug of choice is a phenobarbital intravenous injection because it can be given in high doses if necessary. 

• Standard dosage for acute convulsions: 20–320 mg intramuscular or intravenous injection every six hours as necessary

• Standard dosage for status epilepticus: 15 mg/kg intravenous injection with a second 10 mg/kg intravenous injection 10 minutes later if necessary

Phenobarbital dosage for sedation

Phenobarbital is a powerful sedative (calming drug). Doctors use it as a short-term treatment for anxiety, tension, alcohol withdrawal, hyperactivity, or right before a medical procedure. As a sedative, phenobarbital doses are usually taken by mouth. Doctors use phenobarbital injections before medical procedures.

• Standard dosage for daytime sedation: 30–120 mg tablets taken in two to three divided doses

• Standard dosage for preoperative sedation: 100–200 mg intramuscular injection given 60–90 minutes before the procedure

Phenobarbital dosage for insomnia

In addition to its sedative properties, phenobarbital is also a powerful hypnotic (a sleep-inducing drug). Doctors sometimes use it as a short-term treatment for insomnia. This may be a last-ditch or rarely used option due to the addictive property of phenobarbital. 

Healthcare providers can use a variety of drugs and other treatments to help people with insomnia sleep. Some are long-term treatments, but others, like phenobarbital, put people to sleep. Taken at bedtime, the drug slows down the brain and makes it easier to fall asleep and stay asleep.

Doses are taken at bedtime by mouth or can be injected by a healthcare professional.

• Standard dosage for insomnia: 100–200 mg taken at bedtime OR 100–320 mg intravenous or intramuscular injection given at bedtime

• Maximum dosage for insomnia: 400 mg daily

Phenobarbital dosage for children

The FDA has approved phenobarbital for use in children as young as newborns. Healthcare professionals use it for seizure disorders, active seizures, febrile seizures (seizures caused by high fever), daytime sedation, and preoperative sedation. 

There are no children’s formulations of phenobarbital, but children who cannot swallow tablets safely can use a liquid oral version or receive injections. For very young children, doctors may use a low-concentration phenobarbital solution sold under the brand name Sezaby.

• Standard phenobarbital dosage for the maintenance treatment of epilepsy in neonates, children, and adolescents: 3–6 mg/kg daily

• Standard phenobarbital dosage for preterm neonatal seizures: 20 mg/kg loading dose followed by a second dose (if necessary) of 10–20 mg/kg 

• Standard phenobarbital dose for term neonatal seizures: 20 mg/kg intravenous injection followed by a second intravenous injection (if necessary) of 20 mg/kg 

• Standard phenobarbital maintenance dose for neonatal seizures: 1.5 mg/kg intravenous injection given every eight hours OR 2.25 mg/kg intravenous injection given every 12 hours 

• Standard phenobarbital dosage for status epilepticus in neonates, children, and adolescents: 15–20 mg/kg as an intravenous injection followed by 5–10 mg/kg intravenous injection every 15 or 30 minutes

• Maximum phenobarbital dosage for status epilepticus in neonates, children, and adolescents: 40 mg/kg

• Standard phenobarbital dosage for daytime sedation in neonates, children, and adolescents: 30–120 mg daily given in two or three divided doses

• Standard phenobarbital dosage for preoperative sedation in neonates, children, and adolescents: 1–3 mg/kg intramuscular or intravenous injection given 60–90 minutes before the procedure

Phenobarbital dosage restrictions

Dosage adjustments may be needed for people with certain medical conditions:

• Hepatically impaired patients: people with liver problems may be started on a lower dose and monitored for side effects. 

  • Severe hepatic impairment: do not use

• Severe renal impairment:

  • In adult patients (creatinine clearance < 10 mL/min): dose interval extended to one dose every 12–16 hours

  • In pediatric patients (creatinine clearance < 10 mL/min): dose reduced by half and dose interval extended to once every 24 hours

  • Dialysis (hemodialysis or parenteral dialysis): doses administered after dialysis

Phenobarbital dosage for pets

Veterinarians primarily use phenobarbital as a first-line treatment to control seizures in epileptic cats and dogs. It’s generally a safe and well-studied drug in veterinary medicine. Vets may sometimes use it as a sedative in dogs or cats.

Veterinarians typically start with a dose of around 2 to 2.5 milligrams per kilogram of body weight to control seizures. Oral doses are given twice daily. The veterinarian may raise the dose until seizures are under control. The veterinarian may need to perform regular blood tests to measure the blood levels of the drug in the pet.

Common side effects include lethargy or hyperactivity in dogs and lethargy in cats.

For active seizures or status epilepticus, veterinarians typically start with a benzodiazepine injection. If that doesn’t work, they may add phenobarbital until the seizures stop.

How to take phenobarbital

Taking phenobarbital is relatively straightforward. However, it is an addictive medication, so the most important advice is to follow the prescriber’s instructions: 

• Follow all the prescriber’s instructions.

• Do not take more than prescribed or use the drug more often than prescribed.

• Doses are typically taken two or three times daily.

• For insomnia, doses are taken at bedtime.

• Phenobarbital doses can be taken with or without food.

• How to take phenobarbital tablets:

  • Swallow the tablet with a drink of water.

  • If swallowing a tablet is difficult, it can be crushed and mixed with food or liquid. Consume the entire mixture immediately.

  • Store phenobarbital tablets at room temperature in a tightly closed container.

• How to take phenobarbital oral solution or elixir:

  • The oral solution and elixir contain alcohol (13% and 15%).

  • Measure doses with a calibrated oral syringe. If one is not supplied with the medicine, ask the pharmacist to provide one.

  • Store phenobarbital oral solution or elixir at room temperature in a tightly closed, light-resistant container.

Phenobarbital dosage FAQs

How long does it take phenobarbital to work?

The effects of phenobarbital are felt about one hour after a dose is taken. 

How long does phenobarbital stay in your system?

With a half-life between 53 and 118 hours, it can take 10 to 20 days to completely clear a dose from the body. Plasma concentrations remain high enough for the drug to be effective for 10 to 12 hours after a dose is taken. 

What happens if I miss a dose of phenobarbital?

Ask the prescriber what to do in case a dose is missed. 

What interacts with phenobarbital?

To avoid possible drug interactions, tell the prescribing healthcare provider and pharmacist about all the prescription drugs, over-the-counter medications, and supplements you take, particularly:

• Drugs that make you sleepy, such as sleeping pills, opioids, benzodiazepines, or allergy medications (antihistamines)

• The antiepileptic drugs phenytoin, sodium valproate, or valproic acid

• Blood thinners such as warfarin

• Blood pressure medications called calcium channel blockers

• Corticosteroids

• Oral contraceptives

• Estrogens

• Monoamine oxidase inhibitors

• The antibiotic doxycycline

• The antifungal drug griseofulvin

What happens when you mix phenobarbital and alcohol?

Do not drink alcohol when taking phenobarbital. Alcohol increases the sedative side effects of phenobarbital, such as drowsiness, dizziness, and impairment.

Is it safe to take phenobarbital during pregnancy?

Phenobarbital can harm a fetus. It can also produce withdrawal symptoms in a newborn when phenobarbital is taken during the third trimester. It is not prohibited, but women who are pregnant or planning to get pregnant should get medical advice from the prescriber.

Is it safe to take phenobarbital while breastfeeding?

Phenobarbital isn’t known to decrease milk production, but small amounts of the drug are found in breast milk and may affect the baby. Mothers should be cautious. Watch for signs of CNS depression, such as excessive sleepiness, decreased muscle tone, or failure to feed.

Who should not take phenobarbital?

Phenobarbital is never prescribed to people with:

• Known hypersensitivity to barbiturates

• Severe liver impairment

• Respiratory or pulmonary problems that involve difficulty breathing or airway obstruction

• Respiratory depression

• Porphyria

Sources

• Generalized tonic-clonic seizure, StatPearls

• Phenobarbital, StatPearls

• Phenobarbital drug summary, Prescriber’s Digital Reference (PDR)

• Phenobarbital elixir prescribing information, DailyMed (NIH National Library of Medicine)

• Phenobarbital liquid prescribing information, DailyMed (NIH National Library of Medicine)

• Phenobarbital sodium injection prescribing information, DailyMed (NIH National Library of Medicine)

• Phenobarbital tablet prescribing information, DailyMed (NIH National Library of Medicine)

• Seizure disorders: part I. classification and diagnosis, Western Journal of Medicine

• Seizure rescue therapies, Epilepsy Foundation

• Sezaby phenobarbital sodium injection prescribing information, DailyMed (NIH National Library of Medicine)

• Status epilepticus, StatPearls