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How and when to take phenytoin

Phenytoin is a prescription medicine. It's important to take it as advised by your doctor.

Dosage and strength

Standard phenytoin tablets come in 100mg strengths and the chewable tablets (Infatabs) are 50mg. Capsules come as 25mg, 50mg, 100mg and 300mg. Phenytoin liquid has 30mg in every 5ml.

The usual dose to treat epilepsy is:

  • adults – 200mg to 500mg a day, taken as 1 or 2 doses
  • children aged 12 to 17 years – 300mg to 400mg, taken as 2 doses
  • children aged up to 11 years – varies depending on their weight

The usual dose to treat trigeminal neuralgia in adults is 300mg to 500mg a day, taken as 1 or 2 doses.

To prevent side effects, your doctor will prescribe a low dose to start with and then increase it over several weeks.

You might need to have blood tests when you start phenytoin or if your dose is changed. This is to make sure your dose is right for you.

Once you find a dose that suits you, it will usually stay the same.

How to take it

You'll usually take phenytoin once or twice a day. You can take it with or without food.

If you take it twice a day, try to space your doses evenly through the day. For example, take it first thing in the morning and in the evening.

How to take standard tablets and capsules

Swallow the tablets or capsules whole with a drink of water, milk or juice. Do not chew them.

How to take chewable tablets

Chew the tablets before swallowing. Do not swallow them whole.

How to take liquid

Liquid phenytoin comes with a plastic syringe or spoon to help you measure your dose. If you do not have a syringe or spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not measure the right amount.

How long to take it for

If you have epilepsy, it's likely that once your condition is under control you'll still need to take phenytoin for many years.

If you have trigeminal neuralgia, you'll keep taking phenytoin for as long as you need, sometimes many years, unless your pain gets better or disappears altogether.

Do not stop taking phenytoin without talking to your doctor first.

If you forget to take it

If you take phenytoin:

  • once a day – take the missed dose as soon as you remember. If it's less than 12 hours before the next dose is due, it's better to skip the missed dose and take your next dose at the usual time
  • twice a day – take the missed dose as soon as you remember. If it's less than 8 hours before the next dose is due, it's better to skip the missed dose and take your next dose at the usual time

Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.

If you have epilepsy, it's important to take this medicine regularly. Missing doses may trigger a seizure.

If you forget doses often, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

If you take too much

Taking too much phenytoin can cause unpleasant side effects.

Urgent advice: Contact 111 for advice now if:

  • you take more than your prescribed dose of phenytoin – even if you do not have any side effects

Go to 111.nhs.uk or call 111

If you need advice for a child under the age of 5 years, call 111.

If you need to go to A&E, do not drive yourself. Get someone to drive you or call for an ambulance.

Take the phenytoin packet or the leaflet inside it, plus any remaining medicine, with you.

Stopping phenytoin

Do not stop taking phenytoin without talking to your doctor first.

If you're taking phenytoin for epilepsy, stopping it suddenly can cause seizures.

Stopping phenytoin should be done very slowly and might take a few months.

If you're taking phenytoin for trigeminal neuralgia and your pain goes away, your doctor will slowly reduce your dose as low as possible, or may even stop it.

If you get a serious side effect, such as a severe skin rash, your doctor may tell you to stop taking phenytoin straight away, even if you have epilepsy.

Page last reviewed: 9 March 2022
Next review due: 9 March 2025