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Phenobarbitone drug information

Patient information A-Z

Drug names

  • Phenobarbitone
  • Phenobarbital


Phenobarbitone is used to reduce the frequency and severity of all types of epileptic seizures. It should not be used to treat absence seizures.

It may be used alone or in combination with other antiepileptic medication.


Phenobarbitone is available in 15mg, 30mg and 60mg tablets.

It also available in a 15mg / 5ml elixir (liquid).

An injectable formulation is available for use in the hospital setting only.

Switching between formulas

It is recommended that once you are prescribed a particular brand or manufacturer’s version of phenobarbitone you should continue to take that particular preparation. Switching of brands or manufacturers should only be done under supervision of your doctor or epilepsy specialist.

Side effects (the list of side effects is not exhaustive, please refer to product literature for full list)

Common side effects include:

  • Listlessness, irritability or sleepiness
  • Visual problems or unusual eye movements
  • Changes in thinking and memory
  • Problems with co-ordination, balance or speech
  • Depression

Less common side effects include:

  • Nausea or vomiting
  • Allergic reaction, such as rash. You should see a doctor as soon as possible if a rash develops
  • Hallucinations
  • A drop in blood pressure
  • Liver problems – your doctor can monitor your liver function with blood tests
  • Anaemia – your doctor will monitor for this with blood tests
  • Hyperactivity – particularly in children and elderly patients

All antiepileptic medication can cause feelings of depression, or severe mood changes. If you experience any of these please contact your specialist

Long term problems with phenobarbitone include:

  • Decreased sex drive
  • Vitamin D deficiency leading to bone problems – you may need to take some supplements prescribed by your doctor if this occurs

In the event of any of these side effects, contact the epilepsy nurse (contact details are at the end of the leaflet), or your GP.


Phenobarbitone reduces the amount of the oral contraceptive pill in your body so that a higher dose pill is required. Even then, contraception may be slightly less effective, especially if there is breakthrough bleeding between periods.

You should consider also using a barrier method, such as condoms until the problem gets rectified. Phenobarbitone does not affect the three monthly contraceptive injection and other methods such as the Mirena coil.

Pregnancy and breastfeeding

Phenobarbitone may harm your baby if taken during pregnancy. Abnormalities include heart disease, problems with the structure of the mouth (cleft palate) and face, spina-bifida, and structural problems with the skull and brain.

Ask to see your specialist to discuss the best treatment before you conceive. Once you are pregnant, it is too late. We recommend you take folic acid 5mg daily twelve weeks prior to conception until the end of weeks 12 of pregnancy.

Phenobarbitone is known to be present in breast milk, it is not recommended for use whilst breastfeeding.

Drug interactions

Phenobarbitone affects the function and level of various drugs including other antiepileptic medication. The extent of this effect varies from person to person depending on genetic factors. Other medication can alter levels of phenobarbitone.

Always tell your doctor or pharmacist that you take phenobarbitone If you need to take over the counter or herbal medication, such as cold remedies always check with your pharmacist that these are safe to be taken with phenobarbitone.

Other useful information

  • Try to take the medicine at the same time every day; it can be taken with or without food.
  • Store phenobarbitone in the original container below 25°C, the elixir should also be protected from light.
  • Keep a record of your seizures when you start any new medication. This will help determine future drug dosages.
  • Do not stop taking phenobarbitone suddenly as this can cause withdrawal seizures. Always withdraw it under supervision and following discussion with your doctor or epilepsy specialist nurse.
  • If you miss a dose, take it as soon as you remember but do not take two doses at the same time or within six hours of each other.

Contact details

If you have any questions or concerns, please contact the epilepsy nurse specialist on 01223 217992.

This information is not exhaustive. Please refer to the patient information leaflet prepared by the manufacturer of your drug which can be found in the medicine packaging.

We are smoke-free

Smoking is not allowed anywhere on the hospital campus. For advice and support in quitting, contact your GP or the free NHS stop smoking helpline on 0800 169 0 169.

Other formats

Help accessing this information in other formats is available. To find out more about the services we provide, please visit our patient information help page (see link below) or telephone 01223 256998.

Contact us

Cambridge University Hospitals
NHS Foundation Trust
Hills Road, Cambridge

Telephone +44 (0)1223 245151