About phenytoin

Phenytoin is a medicine used to treat epilepsy.

It can also be used to treat trigeminal neuralgia, a type of nerve pain that affects your face.

Phenytoin is available on prescription. It comes as tablets that can be chewed or dissolved in water, capsules and a liquid that you swallow.

Key facts

  • It's usual to take phenytoin once or twice a day. You can take it with or without food.
  • The most common side effects of phenytoin are headaches and feeling dizzy.
  • It takes around 4 weeks for phenytoin to work.
  • If you have epilepsy, it's best to stay on the same brand of phenytoin, as other brands may not work as well for you.
  • If you take it for trigeminal neuralgia, most people do not have to stay on the same brand.

Who can and cannot take phenytoin

Phenytoin can be taken by adults and children of any age.

Phenytoin is not suitable for some people.

To make sure phenytoin is safe for you, tell your doctor if you:

  • have ever had an allergic reaction to phenytoin or other medicines in the past
  • have liver or kidney problems
  • have a blood disorder called porphyria
  • cannot have alcohol – phenytoin liquid contains a small amount of alcohol
  • have had a rash caused by phenytoin in the past
  • are pregnant or planning to become pregnant
  • have ever had an allergic reaction to carmoisine (E122) or sunset yellow (E110) – some brands of phenytoin liquid and tablets contain these
  • have an intolerance to or cannot absorb some sugars – some brands of phenytoin tablets contain sugars

How and when to take phenytoin

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


The usual dose of phenytoin to treat epilepsy in:

  • adults is between 200mg and 500mg a day, taken as 1 or 2 doses
  • children (aged 12 to 17 years) is between 300mg and 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 between 300mg and 500mg a day, taken as 1 or 2 doses.

How to take it

It's usual to 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, first thing in the morning and in the evening.

Capsules and tablets – swallow whole with a glass of water, milk or juice. Do not chew them.

Chewable tablets – can be swallowed whole with a glass of water or chewed.

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

Will my dose go up or down?

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.

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

How long will I 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, phenytoin will be continued for as long as necessary, sometimes many years, unless your pain gets better or disappears altogether.

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

What if I forget to take it?

If you take phenytoin and miss a dose:

  • 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 leave out the missed dose and take your next dose as normal.
  • 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 leave out the missed dose and take your next dose as normal.

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 fit (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.

What if I take too much?

Taking too much phenytoin can cause unpleasant side effects.

Call your local emergency service if:

  • you take too much phenytoin – even if you do not have any side effects

If you need to go to a hospital, 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.

Side effects

Like all medicines, phenytoin can cause side effects, although not everyone gets them.

Skin rashes

It's common to get a skin rash with phenytoin. Most skin rashes are not serious.

But if you develop a skin rash or redness, tell a doctor straight away, as this can develop into a life-threatening skin condition called Stevens-Johnson syndrome.

Stevens-Johnson syndrome is a rare side effect of phenytoin.

It causes flu-like symptoms, followed by a red or purple rash that spreads and forms blisters. The affected skin eventually dies and peels off.

It's more likely to happen in the first 8 weeks of starting phenytoin, or when the dose is increased too quickly.

It can also happen if phenytoin is stopped suddenly for a few days and then restarted at the same dose as before, without reducing the dose and then increasing it slowly again.

Stevens-Johnson syndrome is more common in:

  • children
  • people who have developed a rash before with a different epilepsy medicine
  • people who are allergic to an antibiotic called trimethoprim
  • people also taking a medicine called sodium valproate

To help prevent the chance of you getting a rash that could be confused with Stevens-Johnson syndrome, it's best to not start any new medicines, foods or products during the first 3 months of treatment with phenytoin.

It's also best to not start phenytoin within 2 weeks of a viral infection, vaccination or rash caused by something else.

Common side effects

These common side effects may happen in more than 1 in 100 people. They're usually mild and go away by themselves.

Keep taking the medicine and talk to your doctor or pharmacist if these side effects bother you or do not go away:

  • headaches
  • feeling drowsy, sleepy or dizzy
  • feeling nervous, unsteady or shaky
  • feeling or being sick (nausea or vomiting)
  • constipation
  • sore or swollen gums
  • mild skin rash

Some side effects of phenytoin wear off once your body gets used to the medicine, but it can take several weeks or months.

Serious side effects

Very few people taking phenytoin have serious problems.

Tell your doctor straight away if you have a serious side effect, including:

  • thoughts of harming or killing yourself – a small number of people taking phenytoin have had suicidal thoughts, which can happen after a few weeks of treatment
  • unexpected bruising or bleeding, a high temperature or sore throat – these could be warning signs of a blood disorder
  • a high temperature with swollen glands and a skin rash, sometimes with yellowing of your skin or the whites of your eyes, particularly in the first 2 months of treatment – these may be signs of a hypersensitivity reaction, which is more likely to happen if you're black African-Caribbean or have a weakened immune system
  • a skin rash – this might be a sign of Stevens-Johnson syndrome

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to phenytoin.

These are not all the side effects of phenytoin. For a full list, see the leaflet inside your medicines packet.

How to cope with side effects

What to do about:

  • headaches – make sure you rest and drink plenty of fluids. Do not drink too much alcohol. If it's a problem, ask your pharmacist to recommend a painkiller. Talk to your doctor if your headaches last longer than a week or are severe.
  • feeling drowsy, sleepy or dizzy – as your body gets used to phenytoin, these side effects should wear off. Do not drive, ride a bike or operate machinery until you feel more alert. If they do not go within a week or two, your doctor may reduce your dose or increase it more slowly. If that does not work, you may need to switch to a different medicine.
  • feeling nervous, unsteady or shaky – talk to your doctor if this bothers you. These symptoms can be a sign that the dose is too high for you.
  • feeling or being sick – stick to simple meals and do not eat rich or spicy food. It might help to take your phenytoin after a meal or snack. If you're being sick, try taking small, frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee. Speak to a doctor if your symptoms get worse or last more than week.
  • constipation – eat more high-fibre foods, such as fresh fruit and vegetables, and cereals. Try to drink several glasses of water or another non-alcoholic liquid each day. If you can, try to exercise more regularly – for example, by going on a daily walk or run.
  • sore or swollen gums – brush your teeth and gums twice a day to help stop this happening, and visit your dentist regularly
  • mild skin rash – it may help to take an antihistamine, which you can buy from a pharmacy. Check with the pharmacist to see what type is suitable for you. Use lukewarm water rather than hot in the shower. Contact your doctor if it spreads, gets very itchy or lasts more than a couple of days.

Pregnancy and breastfeeding

Phenytoin and pregnancy

Phenytoin has been linked to an increased risk of problems for your unborn baby.

You'll usually only be advised to take it if your doctor thinks the benefits of the medicine outweigh the risks.

It's important that you and your baby stay well during pregnancy.

If you take phenytoin and become pregnant, tell your doctor or nurse straight away.

If you have epilepsy, it's very important that it's treated during pregnancy as seizures can harm you and your unborn baby.

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

If you're pregnant or trying to get pregnant and taking phenytoin, you're recommended to take a higher dose of folic acid, a vitamin that helps your baby grow normally.

Your doctor might prescribe a high dose of 5mg a day for you to take when you're trying to get pregnant and during the first 12 weeks of pregnancy.

Phenytoin and breastfeeding

If your doctor or health visitor says your baby is healthy, phenytoin can be taken while you're breastfeeding.

Phenytoin passes into breast milk in small amounts. There have been some reports of side effects in breastfed babies, but in most cases babies do not get any side effects.

It's important to keep taking phenytoin to keep you well. Do not stop taking it without talking to your doctor.

Breastfeeding will also benefit both you and your baby.

If you notice that your baby's not feeding as well as usual, seems unusually sleepy, or you have any other concerns about them, talk to your pharmacist, health visitor or doctor as soon as possible.

Cautions with other medicines

There are lots of medicines that can affect phenytoin and increase the chances of you having side effects.

If you need to take these medicines, your doctor may change your dose of phenytoin.

Tell your doctor if you're taking:

  • other medicines used to treat epilepsy, such as carbamazepine, clonazepam, lamotrigine, phenobarbital, sodium valproate, topiramate, oxcarbazepine, ethosuximide and vigabatrin
  • medicines used to treat HIV
  • hormonal contraceptives or hormone replacement therapy (HRT)
  • medicines used for heart and circulation problems like digoxin, amiodarone, furosemide, warfarin, diltiazem, nifedipine, apixaban and ticagrelor
  • medicines used to treat fungal infections like fluconazole, itraconazole and miconazole
  • medicines used for tuberculosis and other infections like isoniazid, rifampicin, sulfamethoxazole-trimethoprim, doxycycline and ciprofloxacin
  • medicines used for stomach problems like omeprazole, sucralfate, ranitidine and some antacids
  • theophylline, a medicine to help with breathing problems
  • medicines used for sleeplessness and mental health problems like chlordiazepoxide, clozapine, clobazam, diazepam, disufiram, fluoxetine, quetiapine, trazodone and amitriptyline
  • medicines used for transplants such as ciclosporin and tacrolimus
  • medicines used for cancer
  • bupropion, a medicine to help you stop smoking

This is not a full list of medicines that can affect phenytoin. For a full list, see the leaflet inside your medicines packet.

Mixing phenytoin with herbal remedies and supplements

There may be a problem taking some herbal remedies and supplements alongside phenytoin.

Do not take St John's wort with phenytoin, as it decreases the amount of phenytoin in your blood.

If you already take it, talk to your doctor before stopping St John's wort as they may need to increase your phenytoin dose.

Common questions about phenytoin