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Phenytoin drug information

Patient information A-Z

This leaflet is for patients who have been prescribed phenytoin for epilepsy.

Purpose of the drug

Phenytoin is an antiepileptic medication that is used to reduce the frequency of seizures. It is used for tonic-clonic and focal seizures. Phenytoin may also be used for prevention and treatment of seizures during or following neurosurgery or after severe head injury.

Drug brand names

  • Epanutin.
  • Phenytoin – various manufacturers.


  • Capsules are available as 25mg, 50mg, 100mg and 300mg strengths.
  • Tablets are available as 100mg strength.
  • *Chewable tablets (Epanutin Infatabs) are available as 50mg strength.
  • *Suspension (30mg/5ml).
  • Intravenous formulations for use in hospital only.

*When switching between certain formulations of phenytoin care must be taken as not all are equivalent. Please seek advice from your pharmacist, neurologist or specialist nurse if you are ever unsure.

Taking Phenytoin:

  • Phenytoin is best taken with food if possible. Tablets (non chewable) and capsules should be swallowed whole. Infatabs may be chewed.
  • If receiving artificial feed eg NG or PEG feeds ensure a two hour gap between feed and taking phenytoin.
  • Do not stop taking your medication suddenly or without consultation with your doctor.
  • Store phenytoin as per the instructions on the package leaflet.
  • Keep a record of your seizures when you start any new medication. This will help to determine future drug dosages.
  • If you miss a dose, take it as soon as you remember but do not take two doses at the same time.

Switching between brands

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

Drug interactions

  • Paracetamol can be taken with phenytoin.
  • Phenytoin can alter levels of a variety of medication; also other medication could alter the effectiveness of phenytoin.
  • Always remind your doctor or pharmacist that you take phenytoin when new medication is discussed or started.
  • Herbal medications should be avoided where possible.

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

In our experience side effects are often dose related and a reduction in dose may help to ease the side effect. We may need to take a blood test to check that the phenytoin dose you are taking is correct for you.

Common side effects include:

  • Dizziness and drowsiness.
  • Loss of concentration.
  • Unsteadiness when walking or tremor.
  • Nausea, vomiting, constipation, loss of appetite.
  • Problems with gums, particularly growth of gums over the teeth. The best way to minimise this is to practise good dental hygiene and to have regular dentist appointments.

Rare side effects include:

  • Rash, any rash should be reported to a health professional as soon as possible.
  • Movement problems.
  • Tingling or changes in sensation in the hands and/or feet.
  • Thickening of the skin or acne.
  • Excessive hair growth.
  • All antiepileptic medication can cause feelings of depression, or severe mood changes. If you experience any of these please contact your specialist.
  • Medically serious side effects are very rare but may affect the blood or the liver. Blood tests will be performed where appropriate.

If levels of phenytoin in the body are too high the following side effects are more likely and should be reported to a medical professional as soon as possible:

  • Double vision or unusual eye movements.
  • Slurred speech or balance problems.
  • Confusion, delirium or psychosis.
  • High blood sugar levels.

The following other side effects should also be reported to a medical professional as soon as possible:

  • Fever, rash or mouth ulcers.
  • Unusual or unexplained bleeding, bruising or a sore throat.
  • Increase in seizure frequency.
  • Yellowing of the skin or abdominal pain.


Phenytoin is a very effective medication, but usually works at its best if levels are kept within a certain range.

The levels in your body can alter greatly with only a small change in dose. For this reason it is important that you attend for blood tests when required.


Phenytoin reduces the amount of oral contraceptive in your body. A higher dose of the oral contraceptive is required, or change of contraceptive regimen. Even with these changes, contraception may be slightly less effective, especially if there is breakthrough bleeding in between periods. You may need to consider using barrier methods of contraception eg condoms.

Pregnancy and breast feeding

The risks to your baby if phenytoin is taken during pregnancy include; cleft lip or palate, heart malformations, abnormalities of the skull, growth problems and under-developed finger and toe nails.

You should ask to see your specialist to discuss treatment before you conceive. Once you are pregnant it is too late to make changes.

We recommend you take folic acid 5mg daily from at least twelve weeks prior to conception until the end of week 12 of pregnancy in order to reduce this risk.

If your doctor considers it necessary for you to take phenytoin during pregnancy you will need to have blood tests to ensure that the dose you are prescribed is still appropriate for you.

The manufacturer advises that phenytoin should not be taken whilst breast feeding. If use is unavoidable the infant should be monitored for sedation, feeding difficulties, adequate weight gain and progress towards developmental milestones.

Contact details

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

The information on this leaflet 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