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Phenobarbital: What It Is Used For And How It Works

Phenobarbital uses and mechanism of action

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Phenobarbital uses are highly varied, giving the medication significant therapeutic applications. For many individuals, phenobarb is key to living a better life. However, those taking it need to understand both its uses and the phenobarbital mechanism of action.

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Table of Contents

How Phenobarbital Works

The phenobarbital mechanism of action is central to the function of the medication. All of what phenobarbital does comes down to how it acts on the body, and that is what is described by the mechanism of action.

The phenobarbitone MOA is not fully understood. What is known is that the phenobarbital MOA begins with the medication acting on GABAA receptors. This increases synaptic inhibition. It acts on the Gamma-aminobutyric acid receptor subunit alpha-1 as a potentiator. It also acts as an antagonist on the following:

  • Neuronal acetylcholine receptor subunit alpha-4
  • Neuronal acetylcholine receptor subunit alpha-7
  • Glutamate receptor 2
  • Glutamate receptor ionotropic, kainate 2
  • NMDA receptor

Finally, it serves as an activator for the Nuclear receptor subfamily 1 group I member 2.

The molecular formula of Luminal (phenobarb) is C12H12N2O3. It is absorbed in two phases, with the salts being absorbed by the body faster than the acids. Phenobarbital metabolism is done through the liver, primarily via CYP2C19. Through metabolization, it produces the following metabolites:

  • p-Hydroxyphenobarbital  Phenobarbital O-sulfate
  • p-Hydroxyphenobarbital  Phenobarbital O-glucuronide
Effects are felt within about 30 minutes of taking the drug. The half-life of phenobarb is significant, which means it continues to impact the body for days after it is taken.

What Phenobarbital Is Used For

Phenobarb is perhaps best known as a drug used for lethal injection. However, what phenobarbital is used for extends far beyond that. The phenobarbital schedule is Schedule IV, meaning that while there is potential for abuse, it has acceptable therapeutic applications that outweigh the risks in many cases. Phenobarbitone level will vary based on the application.


Phenobarbital for seizures is one of the oldest uses of the medication. Phenobarbital seizure medication is the earliest that remains on the market. When used for this application, it is usually taken just once per day, sometimes in conjunction with other medications, and is explicitly meant for tonic-clonic seizures.

It is considered to be highly useful to use phenobarb for seizures. However, in many countries, it has fallen out of use for this.

The medication is known to have numerous serious side effects, leaving the World Health Organization to recommend it only be used to treat seizures in the developing world due to the low cost of the drug and its limited drug interactions, allowing it to be a safer choice in areas where medical access is restricted.


Phenobarbital use originally was as a sedative. While it can be used for short-term sedation, such as for emergency treatment, in surgery, or to calm someone have a psychiatric episode, it is most famous as a long-term sedative for individuals in intensive care. This is because it is longer acting than many other tranquilizers on the market, allowing caregivers to give less medication to the patient, which is both healthier and more cost-effective. The primary downside is that dependence can develop, but in many cases, the benefits are worth the risk.


In some cases, phenobarbital use may be prescribed for sudden-onset insomnia. When it is prescribed for this condition, it is only done for the short-term due to the risk of addiction. For those who have long-term insomnia, other medications must be used to avoid addiction and the side effects that emerge with ongoing use. Users take a single dose before bed, for seven days or less. If insomnia persists, other therapies will be considered.

woman suffering from insomnia

Alcohol Withdrawal

Another application is phenobarbital for alcohol withdrawal. In this case, it is used to sedate the patient so that they either do not experience delirium tremens, or so that their experience of it is lessened. The patient is given the medication as often as needed to keep them at the desired level of sedation.

It is common to use barbiturates for this application, and phenobarb has proven to be better than most in its drug class for this use. However, it is not yet clear if using phenobarbital for alcohol withdrawal can harm the individual ending addiction.

Benzodiazepine Withdrawal

Another of the uses for phenobarbital is in helping benzodiazepine addicts get clean. The standard procedure is to give the patient the medication for three days, tapering as the days go on, stopping use at the end of day three. In limited studies, it is effective, with little adverse side effects beyond patients experiencing too much sedation from the drug. Still, as with alcohol, there are questions regarding if it harms staying clean.

Opiate Withdrawal

In some cases, rehabilitation centers will use phenobarbital for opiate withdrawal. While it does not eliminate the symptoms, it does sedate the patient so that they are less aware of them. Some studies have shown that it can speed up the metabolism of drugs, which might push them out of the system sooner.

However, this does not prevent withdrawal and may even cause it to happen faster.

Ultimately, it is not a first-choice drug for opiate withdrawal as there are other medications with sedative properties that have fewer side effects.

Medically Induced Comas

Barbiturates, including phenobarb, can be used to cause medically induced comas. When creating a phenobarbital coma, patients are given a steady drip of the medication to keep them fully sedated. While this approach works, it is not favored. There are significant complications that emerge from phenobarb comas. The long-term effects can impact the survival of the patient, making other medications better options.

woman in coma


The use of phenobarbitone injection in executions as well as doctor-assisted suicide is the most well-known use of the drug at the current time. Because the phenobarbital cost is low and the drug is highly effective, many state governments that still allow for capital punishment prefer it. For it to be effective, it must be given in very high doses and is often accompanied by other medications, creating a drug cocktail. Phenobarbital euthanasia is also used with animals at some vet practices.

Recreational Use

Sometimes, phenobarbital tablets are used recreationally.

This is extremely dangerous, and non-medical use is considered an abuse. This medication can easily become deadly. People using phenobarb recreationally should seek help stopping their use before it is too late.

How Phenobarbital Should Be Used

Because phenobarb has the potential to be deadly, it is vital that anyone using it carefully follows the instructions set out by their doctor. They also should read the phenobarbital package insert that comes with their prescription to make sure they are fully informed about the risks that come with use.

In most cases, phenobarb is taken just once a day, but if using lower phenobarbital strength medication, patients may be instructed to take it twice per day. It can be taken with or without food, though some patients may feel sick if they take it on an empty stomach.

The proper time of day for taking phenobarb will depend on the reason the patient is using it. If they are taking it for seizures, they may need to take the medication at the start of the day. Those treating insomnia can take it before bed. In some cases, there may be no ideal time beyond what works best for the individual.

Patients who are taking phenobarb should check with their doctor before starting or stopping any medications. This drug does not have a lot of known interaction, but some are dangerous. Caution should always be exercised.

Who Should Not Use Phenobarbital

People who have had allergic reactions to other barbiturates should not use phenobarb. Pregnant and nursing women should avoid its use due to phenobarbital nursing implications. Anyone who is particularly sensitive to the common side effects of phenobarbital should investigate suitable alternatives before taking the medication.

People with a history of drug abuse should not be prescribed the medicine due to phenobarbital high. Anyone with mental illness or depression should either take another medication or be monitored very closely while on phenobarb. People with liver disease cannot take the medicine as the liver metabolizes it. Finally, those with sleep apnea should avoid the medication as it slows respiration and can lead to death.

Anyone who is using phenobarbital recreationally should seek help in getting clean. Because the drug carries such a high risk of overdose and death, the high it offers is simply not worth the price the user could end up paying. Drug rehab centers know how to address barbiturate addiction and get users clean safely.

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Roger Weiss

About Author

Roger Weiss, MD

Dr. Roger Weiss is a practicing mental health specialist at the hospital. Dr. Weiss combines his clinical practice and medical writing career since 2009. Apart from these activities, Dr. Weiss also delivers lectures for youth, former addicts, and everyone interested in topics such as substance abuse and treatment.


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