Phénobarbital
enzyme-inducing antiepileptic
Third-party optionBy the Layer Nutrition editorial team · How the atlas classifies the 664 molecules
What this medicine is for
Pharmacologically, Phenobarbital relates to the category “enzyme-inducing antiepileptic”. Antiepileptics stabilize the electrical activity of the brain to prevent seizures; they are taken regularly and are not stopped abruptly.
For Phenobarbital, in practice: Treats epilepsy. It is most often a long-term treatment, taken at set times over long periods; its benefit is judged over time, not by how you feel on a given day.
Under which brands it is found
In France, about 4 reimbursable presentations containing Phenobarbital are recorded. It is found under several brand names and as generics.
- Alepsal
- Gardenal
- Phenobarbital Richard
- and other presentations
Original brand-name products and generics deliver the same active substance; the choice between them is made with your doctor or pharmacist.
On your plate: vitamin D and folate
Long-term treatment with phenobarbital lowers the body's vitamin D: by slowing its activation in the liver, it reduces the usable form. Food helps maintain these intakes, day after day, alongside your treatment.
Oily fish are among the best sources of vitamin D: salmon, mackerel, tuna. Egg yolk, cheese and mushrooms, especially when they have been exposed to UV light, also provide some.
Phenobarbital also interferes with the metabolism of folic acid. Folate is concentrated in dark green leafy vegetables, legumes such as beans and dried peas, and citrus fruits. Building meals around these foods covers a good part of your needs.
How phenobarbital works
Phenobarbital belongs to the barbiturate family, non-selective depressants of the central nervous system. It acts on the neuron's GABA-A receptor: by binding to it, it keeps the associated chloride channel open. More chloride then enters the cell, which hyperpolarizes its membrane. The neuron becomes harder to excite, and the threshold for triggering a seizure rises.
Like all barbiturates, phenobarbital slows brain activity more broadly, so its action is not limited to seizure control. This dual dimension explains both its anticonvulsant efficacy and part of its effects. The desired benefit lies in a balance: a dose high enough to prevent seizures, measured enough to limit sedation.
Why phenobarbital is prescribed
Phenobarbital is an anticonvulsant. It is indicated for the treatment of generalized epileptic seizures, which affect a wide area of the brain from the outset, and partial seizures, which arise from a localized focus. By raising the firing threshold of neurons, it spaces out and reduces the occurrence of these seizures.
The doctor chooses the dose according to the person, their age and the observed response. The aim is to prevent seizures while keeping the effects on alertness at an acceptable level. The response is assessed over time, because the effect depends closely on the concentration reached in the blood.
How to take it day to day
Phenobarbital is taken orally, as a tablet or a solution, one to three times a day depending on the prescription. Unless the doctor advises otherwise, it is taken with no particular constraint regarding meals, and the patient keeps their usual diet. Regular dosing is essential to maintain a stable concentration in the blood.
Its plasma half-life is very long in adults, from 53 to 118 hours, with an average of 79 hours. The medicine is therefore eliminated slowly, which makes a single daily dose possible and explains why the balance settles over several weeks. This inertia has a practical consequence: any dose change and any discontinuation are decided with the doctor, never alone.
The effects to know about
The most common adverse effect is drowsiness, reported in more than 1 patient in 100. Other neurological and psychiatric disorders are less frequent, in fewer than 1 patient in 100: agitation, confusion, ataxia (coordination problems), central nervous system depression, nightmares, nervousness, hallucinations, insomnia, anxiety and dizziness. These effects stem from the molecule's depressant action on the brain and often depend on the dose.
More rarely, severe skin reactions can occur, such as Stevens-Johnson syndrome and toxic epidermal necrolysis (Lyell's syndrome). They remain exceptional, but require immediate medical attention in the event of a widespread rash, blisters or involvement of the mucous membranes. Recognizing these signs allows a rapid response.
A powerful enzyme inducer
Phenobarbital is a powerful inducer of liver enzymes, the cytochromes P450. In practical terms, it pushes the liver to make more of these enzymes, which then break down many other medicines more quickly. Affected are oral contraceptives, anticoagulants, theophylline and corticosteroids, whose effect may decrease. With dicoumarol, the plasma concentration falls and the anticoagulant activity is reduced, which may require adjusting the dose.
This induction has an important consequence in women: an oral contraceptive can fail, and pregnancies under contraception have been reported in patients treated with antiepileptics. If a pregnancy is planned, folic acid supplementation (5 mg per day) is recommended before conception and during the first trimester. Maternal vitamin K supplementation before delivery has also been proposed, but the increased bleeding risk in the newborn remains debated, with recent data not confirming it.
Blood concentration, bone and monitoring
The efficacy and safety of phenobarbital depend on its concentration in the blood. The useful range lies between 10 and 40 micrograms per milliliter; above 40, the patient enters the toxic zone. This is why the doctor may request blood measurements to adjust the dosage and stay within this window.
Long-term treatment can also affect the bone. Phenobarbital slows the 25-hydroxylation of vitamin D3 in the liver, by reducing the expression of the enzymes CYP27A1 and CYP2D25, which decreases the activation of vitamin D and can lead to a deficiency, or even to osteomalacia. Enzyme-inducing antiepileptics lower circulating 25-hydroxyvitamin D: a meta-analysis found an average of 21.8 ng/mL under carbamazepine versus 28.0 ng/mL in controls. Regular measurement of vitamin D is therefore part of the monitoring of treated people with epilepsy.
Sources
- Phénobarbital
- Phénobarbital, notice de la solution orale
- Phénobarbital, information médicament
- Phenobarbital suppresses vitamin D3 25-hydroxylase expression (Hosseinpour et al.)
- Vitamin D prophylaxis in persons with epilepsy? (Sourbron et al.)
- Impact of carbamazepine on vitamin D levels: a meta-analysis (LoPinto-Khoury et al.)
- Treatment of epilepsy in pregnancy (Nulman et al.)
- Does vitamin K prophylaxis prevent bleeding in neonates exposed to enzyme-inducing antiepileptic drugs in utero? (Rezvani & Koren)
- Vitamine D
- Acide folique dans l'alimentation
Related molecules
To explore in the « Neurology and mind » family and its neighbours.
See the whole « Neurology and mind » family · Back to the atlas
Supplements to consider
Beyond the plate, a single isolated nutrient can be worth discussing, to confirm with your doctor or pharmacist. Here is which one and in which form.
Vitamin D3 (cholecalciferol)
Vitamin D is a fat-soluble vitamin. The D3 form (cholecalciferol) is the one adopted by most guidelines. It is better absorbed when taken during a meal that contains fats.
Preferred form D3 (cholecalciferol), during a fatty meal
Frequently asked questions
What is phenobarbital used for?
Phenobarbital is an antiepileptic in the barbiturate family, prescribed for certain forms of epilepsy and as an anticonvulsant. It is also a powerful enzyme inducer, which alters the action of many other medicines. For the indication that concerns you, refer to the package leaflet and to your doctor or pharmacist.
What are the side effects of phenobarbital?
The leaflet notably mentions drowsiness, sedation, a general slowing down, skin rashes and a risk of dependence. The full list and the frequency of these effects are set out in the medicine's package leaflet. If you experience a bothersome or unexpected effect, talk to your doctor or pharmacist.
Can you stop phenobarbital suddenly?
Stopping phenobarbital abruptly carries a risk of seizures and a withdrawal syndrome, and is never a decision to make on your own. Any change in dose or any discontinuation is done together with your doctor, who adjusts the course of action. Refer to the package leaflet for how to proceed.
Does phenobarbital interact with birth control or blood thinners?
As an enzyme inducer, phenobarbital can reduce the effectiveness of several treatments, including some hormonal contraceptives and blood thinners. Tell your doctor or pharmacist about all your medicines, including those taken without a prescription. The interactions are detailed in the package leaflet.
Can you drink alcohol while taking phenobarbital?
The leaflet advises against alcohol during treatment with phenobarbital, as it can increase drowsiness and sedation. For your specific situation, seek advice from your doctor or pharmacist.
Phenobarbital and vitamin D: what to watch for during long-term treatment?
Studies link enzyme-inducing antiepileptics, including phenobarbital, to a lower vitamin D status, with one proposed mechanism being an accelerated breakdown of the vitamin in the body. The active form to consider is vitamin D3, or cholecalciferol, the best-studied form for vitamin status. Monitoring relies on a blood test of 25-hydroxy-vitamin D, to be interpreted with your doctor, who judges whether supplementation is worthwhile.
Ces réponses portent sur le volet nutritionnel. Pour toute question sur votre médicament, la notice et votre médecin ou pharmacien font foi.