How Long Does Phenobarbital Stay in Your System?

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Phenobarbital is a long-acting barbiturate typically prescribed to control seizures. In some circumstances, it is used in treating withdrawal and for other purposes. Knowing how it acts in your system and for how long can help you understand the precautions to take to avoid dangerous drug interactions and possible overdoses.

How Phenobarbital Acts in Your System

Phenobarbital can be administered as an injection, a liquid elixir, or tablets. Each has their own speed of taking effect and appropriate dosages. Tablets or elixirs begin to act in about 60 minutes, and their duration lasts for 10 to 12 hours, depending on the dosage and individual metabolism.

The plasma half-life of phenobarbital in adults is an average of about 79 hours and 110 hours in children. That means that only half of the active ingredients have stopped exerting their effect in that time. It takes about 4 to 5 half-lives for drugs to be fully eliminated from your system.

Phenobarbital is metabolized by the liver and excreted in the urine. It can be detected in the urine for up to 15 days after a dose. If you take a urine drug screen while on phenobarbital, it will likely test positive for barbiturates.

Phenobarbital can interact with many prescription and nonprescription medications, supplements, and herbal products while it is in your system. It is important to talk to your doctor about all of your medications to avoid these interactions, including ones you might start or stop taking. Some drugs of special concern with phenobarbital include blood thinners, Antabuse, Vibramycin, Fulvicin, hormone replacement therapy (HRT), monoamine oxidase (MAO) inhibitors, oral steroids, sedatives, sleeping pills, tranquilizers, and medications for anxiety, depression, pain, asthma, colds, and allergies, and some medications for seizures like phenytoin (Dilantin) and valproate (Depakene).

Don't drink alcohol while phenobarbital is in your system as it can worsen side effects. Phenobarbital can harm a fetus, so it shouldn't be used if you plan to become pregnant, and it shouldn't be used if you are breastfeeding. Birth control pills and other hormonal contraceptives may be less effective when you are taking phenobarbital, and you may need to use a different form of birth control.

Take Phenobarbital Only as Prescribed

One reason to know how long phenobarbital remains in your system is that it is addictive which means taking larger doses, taking it more often, or taking it longer than prescribed could increase the risk of addiction.

Because phenobarbital can become habit-forming, you should not take the medication if you drink or have ever drunk large amounts of alcohol, used illicit drugs, or overused or used for non-medical purposes any prescription medications.

Don't Stop Taking Phenobarbital Suddenly

Also, do not stop taking phenobarbital suddenly without consulting your healthcare provider. Stopping suddenly can cause you to experience withdrawal symptoms which can include:

  • Anxiety
  • Muscle twitching
  • Uncontrollable shaking of a part of the body
  • Weakness
  • Dizziness
  • Changes in vision
  • Nausea
  • Vomiting
  • Seizures
  • Confusion
  • Difficulty falling asleep or staying asleep
  • Fainting

If you wish to stop taking phenobarbital, your doctor will probably gradually decrease your dosage.


Symptoms of phenobarbital overdose include heart failure, low blood pressure, weak pulse, uncontrollable movements of the eyes, loss of coordination, drowsiness, slowed breathing, confusion, delirium, headache, sleepiness, coma, large blisters, and in rare cases, death.

If you think someone has had a phenobarbital overdose, call the poison control helpline at 1-800-222-1222. If the person has collapsed or is not breathing, call 911 immediately.

3 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. MedlinePlus. Phenobarbital.

  2. Reddy DS, Perumal D, Golub V, Habib A, Kuruba R, Wu X. Phenobarbital as alternate anticonvulsant for organophosphate‐induced benzodiazepine‐refractory status epilepticus and neuronal injuryEpilepsia Open. 2020;5:198-212. doi:10.1002/epi4.12389

  3. U.S. National Library of Medicine. PHENOBARBITAL- phenobarbital tablet.

By Buddy T
Buddy T is an anonymous writer and founding member of the Online Al-Anon Outreach Committee with decades of experience writing about alcoholism.