Phenobarbital belongs to a group of medicines called barbiturates. It is used to treat epilepsy and works by stabilising electrical activity in the brain.

What is phenobarbital used for?

  • Epilepsy.
  • Phenobarbital is available as tablets, elixir (liquid) and injection. The tablets and elixir are taken regularly to help prevent fits. The injection is used to control repeated fitting, when consciousness is not regained between seizures. This condition is called status epilepticus.

How does phenobarbital work?

  • Phenobarbital belongs to a group of medicines called barbiturates. It is used to treat epilepsy and works by stabilising electrical activity in the brain.
  • The brain and nerves are made up of many nerve cells that communicate with each other through electrical signals. These signals must be carefully regulated for the brain and nerves to function properly. When abnormally rapid and repetitive electrical signals are released in the brain, the brain becomes over-stimulated and normal function is disturbed.
This can result in fits or seizures.
  • Phenobarbital prevents epileptic fits by preventing the excessive electrical activity in the brain. It is thought to achieve this by affecting certain neurotransmitters in the brain.
  • Neurotransmitters are chemicals that are stored in nerve cells and are involved in transmitting messages between the nerve cells. GABA is a neurotransmitter that acts as a natural 'nerve-calming' agent. It helps keep the nerve activity in the brain in balance. Glutamate is a neurotransmitter that acts as a natural 'nerve-exciting' agent. It is released when electrical signals build up in nerve cells and subsequently excites more nerve cells. It is thought to play a key role in causing epileptic seizures.
  • Phenobarbital increases the activity of GABA and decreases the activity of glutamate in the brain. These actions help stabilise the electrical activity in the brain and prevent epileptic fits.
  • How do I take phenobarbital?

    • The dose of this medicine that is prescribed will vary from person to person. It is important to follow the instructions given by your doctor. These will also be printed on the dispensing label that your pharmacist has put on the packet of medicine. If you are unsure about anything ask your doctor or pharmacist for advice.
    • Phenobarbital is usually taken once a day at bedtime because it can make you sleepy.
    • The tablets and elixir can be taken either with or without food.
    • If you forget to take a dose take it as soon as you remember, unless it is time for your next dose. In this case, leave out the forgotten dose and take your next dose as usual when it is due. Do not take a double dose to make up for a missed dose.
    • If you have epilepsy it is important to take your medication regularly, as directed by your doctor, because missing doses can trigger seizures in some people. If you have trouble remembering to take your medicine you should ask your pharmacist for advice. You may find a pill reminder box helpful.
    • You should not suddenly stop taking this medicine unless your doctor tells you otherwise, as suddenly stopping treatment is likely to make your seizures return or get worse. It may also result in withdrawal symptoms such as difficulty sleeping (insomnia), anxiety, tremor, dizziness, nausea and delirium.

    Important information about phenobarbital

    • This medication may cause drowsiness. If affected do not drive or operate machinery.
    • You should avoid drinking alcohol while you are taking this medicine because it is likely to make any drowsiness worse.
    • This medicine may rarely cause problems with your blood cells. You should consult your doctor immediately if you experience any of the following symptoms, as these may signs of problems with your blood cells: unexplained bruising or bleeding, purple spots, sore throat, mouth ulcers, high temperature (fever), feeling tired or general illness. Your doctor may want you to have a blood test to check your blood cells.
    • This medicine may very rarely cause severe, potentially life-threatening skin reactions (Stevens Johnson syndrome and toxic epidermal necrolysis). If you get a skin rash, blistering or peeling of the skin, ulcers in the mouth, throat, nose, or genitals, red and sore eyes, a fever or swollen glands while taking this medicine, you should consult your doctor urgently. The highest risk of these very rare skin reactions is in the first few weeks of taking phenobarbital.
    • There may be a small increased risk of suicidal thoughts and behaviour in people taking antiepileptic medicines such as phenobarbital for any condition. For this reason, it is very important to seek medical advice if you, or someone else taking this medicine, experience any changes in mood, distressing thoughts, or feelings about suicide or self-harm at any point while taking this medicine. For more information speak to your doctor or pharmacist.
    • Phenobarbital makes hormonal contraceptives that contain oestrogen and/or progesterone (such as the pill or patch) ineffective at preventing pregnancy. It is important for women who could get pregnant to discuss pregnancy and methods of contraception with their doctor before starting treatment with this medicine.

    Phenobarbital should be used with caution in

    • Elderly people.
    • Children.
    • People who are run down, weak or debilitated.
    • People with any breathing problems.
    • People with decreased liver or kidney function.
    • People with a history of drug or alcohol abuse.
    • People who are suffering from senility or dementia.
    • People suffering from severe chronic pain.

    People who don't consume much calcium in their diet, don't get enough vitamin D from exposure to sunlight or are immobilised for long periods of time, for example due to being confined to bed or having a leg in a plaster cast. If you fall into any of these groups your doctor may want you to take a vitamin D supplement if you are taking phenobarbital for a long time. This is because phenobarbital can interfere with the way the body metabolises vitamin D and calcium, which can cause bone thining and weakening, osteomalacia, osteoporosis and an increased risk of fractures. Ask your doctor or pharmacist for more information. If you have a history of osteoporosis or are also taking long-term corticosteroids these are further risk factors for osteoporosis and you should discuss this with your doctor.

    Phenobarbital should not be used in

    • People who are allergic to barbiturate medicines.
    • People with dangerously slow, shallow breathing (severe respiratory depression).
    • People with an inherited blood disorder called acute intermittent porphyria.
    • People with severe kidney disease.
    • People with severe liver disease.

    This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy.

    If you feel you have experienced an allergic reaction, stop using this medicine and inform your doctor or pharmacist immediately.

    Pregnancy and breastfeeding

    Certain medicines should not be used during pregnancy or breastfeeding. However, other medicines may be safely used in pregnancy or breastfeeding providing the benefits to the mother outweigh the risks to the unborn baby. Always inform your doctor if you are pregnant or planning a pregnancy, before using any medicine.

    • It is vital that women with epilepsy receive specialist advice regarding contraception, pregnancy and planning a family before they start treatment for their condition and before they get pregnant. Pregnant women taking antiepileptic medicine have a higher risk of carrying a baby with developmental problems and malformations than other women. This risk increases if more than one antiepileptic is being taken to control seizures. However, if a woman with epilepsy stops treatment because she gets pregnant, this is likely to cause seizures that can potentially be more harmful to both mother and baby than continuing treatment.
    • Clearly it is important to avoid an unplanned pregnancy while taking an antiepileptic medicine by using an effective method of contraception. You will need to discuss this with your doctor, because phenobarbital makes the pill, mini-pill, patch and vaginal ring ineffective at preventing pregnancy. There are various methods that are not affected by phenobarbital; these include the IUD (eg Mirena), contraceptive injection and barrier contraceptives such as condoms. Another option is to take take two pills together to get a higher dose of hormones, however this is not suitable for all women and is also unlicensed. It is important to get advice from your doctor.
    • If you think you could be pregnant while taking this medicine it is vital that you consult your doctor straight away for advice. Don't stop taking this medicine unless your doctor tells you to.
    • Women who decide to try for a baby while taking phenobarbital should start taking folic acid daily as soon as contraception is stopped, as this may reduce the risk of neural tube defects such as spina bifida in the baby. Ask your doctor for advice on the dose to take - it may be recommended that you take 5mg daily.
    • This medicine passes into breast milk and may cause drowsiness in a nursing infant. However, it may be possible to breastfeed your baby if you are taking phenobarbital on its own and your baby was not born prematurely, has no other health issues and is monitored closely. It is important to get medical advice from your doctor before deciding to breastfeed.

    Possible side effects of phenobarbital

    Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects that are known to be associated with this medicine. Just because a side effect is stated here does not mean that all people using this medicine will experience that or any side effect.

    • Drowsiness.
    • Disturbances in behaviour.
    • Unusual excitement.
    • Hyperactivity, particularly in children and elderly people.
    • Difficulty thinking.
    • Memory problems.
    • Seeing or hearing things that are not really there (hallucinations).
    • Depression.
    • Irritability.
    • Slow, shallow breathing (respiratory depression).
    • Low blood pressure (hypotension).
    • Shaky movements and unsteady walk (ataxia).
    • Rapid involuntary movements of the eyes (nystagmus).
    • Inflammation of the liver (hepatitis).
    • Softening of the bones (osteomalacia). See the cautions section above.
    • Disturbances in the normal numbers of blood cells in the blood. See the warning section above.
    • Serious allergic skin reactions such as toxic epidermal necrolysis or Stevens-Johnson syndrome. See the warning section above.

    The side effects listed above may not include all of the side effects reported by the medicine's manufacturer. For more information about any other possible risks associated with this medicine, please read the information provided with the medicine or consult your doctor or pharmacist.

    How can phenobarbital affect other medicines?

    Phenobarbital can interact with many medicines and the dose of phenobarbital or the interacting medicine may need to be altered as a result. It is important to tell your doctor what medicines you are taking, including herbal medicines and non-prescription medicines, before you start phenobarbital. Likewise, once you are taking phenobarbital it is important to check with your doctor or pharmacist before you stop taking any existing medicines, or start taking any new medicines, including herbal medicines and those bought without a prescription.

    There may be an increased risk of drowsiness if phenobarbital is taken in combination with any of the following, which can also cause drowsiness:

    • alcohol
    • benzodiazepines, eg diazepam, temazepam
    • other barbiturates, eg phenobarbital, amobarbital
    • sedating antihistamines, eg chlorphenamine, promethazine, hydroxyzine
    • sleeping tablets, eg zopiclone
    • strong opioid painkillers, eg morphine, codeine (these may also increase the risk of breathing problems)
    • tricyclic antidepressants, eg amitriptyline.

    Phenobarbital may may increase the breakdown of the following medicines by the liver. As this could decrease the level of these medicines in the blood and may make them less effective, your doctor may need to prescribe a larger than normal dose of these:

    • antipsychotics such as aripiprazole, chlorpromazine, clozapine, haloperidol, sertindole, quetiapine and thioridazine
    • aprepitant
    • azole antifungal medicines, eg ketoconazole, itraconazole, voriconazole, posaconazole
    • avanafil
    • boceprevir
    • buprenorphine
    • bupropion
    • calcium-channel blockers, eg felodipine, nifedipine, nimodipine, verapamil
    • certain anti-cancer medicines, including abiraterone, axitinib, bosutinib, cabazitaxel, crizotinib, dabrafenib, etoposide, gefitinib, imatinib, irinotecan, toremifene, vandatenib
    • chloramphenicol
    • ciclosporin
    • clobazam
    • clonazepam
    • cobicistat
    • corticosteroids such as dexamethasone, methylprednisolone, prednisolone
    • disopyramide
    • dolutegravir
    • doxycycline
    • dronedarone
    • elvitegravir
    • eplerenone
    • ethosuximide
    • fentanyl
    • gestrinone
    • griseofulvin
    • ivacaftor
    • lamotrigine
    • lidocaine
    • methadone
    • metoprolol
    • metronidazole
    • mianserin
    • mirtazapine
    • montelukast

    NNRTIs for HIV, eg etravirine, efavirenz (this may also affect the blood level of phenobarbital), rilpivirine, nevirapine oestrogens and progestogens such as those in the contraceptive pill, patch or vaginal ring
    oestrogens and progestogens in HRT (hormone replacement therapy)

    • ondansetron
    • paroxetine
    • pethidine
    • propafenone
    • propranolol
    • protease inhibitors for HIV infection such as abacavir, fosamprenavir, darunavir, indinavir, lopinavir, saquinavir
    • quinidine
    • ranolazine
    • reboxetine
    • rifampicin (rifampicin may also reduce the blood level of phenobarbital)
    • rivaroxaban
    • roflumilast
    • rufinamide
    • sirolimus
    • sofosbuvir
    • tacrolimus
    • telaprevir
    • telithromycin
    • theophylline
    • thyroid hormones such as levothyroxine
    • tiagabine
    • timolol
    • topiramate
    • tricyclic antidepressants, eg amitriptyline
    • tropisetron
    • ulipristal (the morning-after pill containing ulipristal (EllaOne) should not be used for emergency contraception in women taking phenobarbital)
    • vitamin D
    • warfarin and other coumarin anticoagulants
    • zopiclone
    • zonisamide.

    The following medicines may reduce the breakdown of phenobarbital by the liver. As this could increase the level of phenobarbital in your blood and may increase the risk of side effects, your doctor may need to decrease your phenobarbital dose if you are prescribed any of these:

    • carbamazepine
    • felbamate
    • phenytoin (phenobarbital may also affect the blood level of phenytoin)
    • stiripentol
    • sodium valproate.

    The following medicines may increase the breakdown of phenobarbital by the liver. As this could decrease the level of phenobarbital in your blood and may make it less effective, your doctor may need to increase your phenobarbital dose if you are prescribed any of these:

    • acetazolamide (there may be an increased risk of bone softening due to lack of vitamin D (osteomalacia) if acetazolamide is used in combination with primidone)
    • folic acid
    • pyridoxine (large doses)
    • the herbal remedy St John's wort (Hypericum perforatum) - this should not be taken by people who are taking primidone.



    Health Reference: Epilepsy