Experiences with

Phenytoin

Phenytoin is a medicine used to treat epilepsy.

Seizures are bursts of electrical activity in the brain that temporarily affect how it works. Phenytoin slows down these electrical signals to stop seizures.

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

Phenytoin can help to relieve nerve pain by slowing down electrical impulses in the nerves and reducing their ability to transmit pain.

Phenytoin is available on prescription. It comes as standard tablets, tablets that can be chewed (Infatabs), capsules and a liquid that you swallow.

Who can take phenytoin

Most adults and children of any age can take pheyntoin.

Who may not be able to take phenytoin

Phenytoin is not suitable for some people. To make sure it's safe for you, tell your doctor if you:

• have ever had an allergic reaction to phenytoin or any other medicine
• have liver or kidney problems
• have a rare condition called porphyria
• cannot have alcohol – phenytoin liquid contains a small amount of alcohol
• have ever had a rash caused by phenytoin
• are pregnant or planning to get pregnant

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.

• 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. Ask someone to drive you or call 999 and ask for an ambulance.

Bring the phenytoin packet or leaflet inside it, any remaining medicine, and any other medicines you take.

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.

Common side effects

These common side effects of phenytoin may happen in more than 1 in 100 people. They're usually mild and go away by themselves. There are things you can do to help cope with them:

Some side effects of phenytoin wear off once your body gets used to the medicine, but it can take several weeks or months. Talk to your doctor or pharmacist if these side effects bother you or do not go away.

Serious side effects

Skin rashes

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

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 reduce the chance of you getting a rash that could be confused with Stevens-Johnson syndrome, it's best to not try any new medicines or food 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.

• you get a severe rash with flushing, blisters or ulcers – these can be signs of Stevens-Johnson syndrome

Find your nearest A&E

Very few people taking phenytoin have serious problems.

Tell your doctor or call 111 now if you have a serious side effect, including:

• unexpected bruising or bleeding, a high temperature or sore throat – these could be signs of a blood disorder
• a high temperature, swollen glands or a skin rash, sometimes with yellowing of the whites of your eyes or your skin (this may be less noticeable on brown or black skin), 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 of Black African or African-Caribbean origin or have a weakened immune system

• you have thoughts of harming or killing yourself

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

Taking phenytoin for a long time can:

• decrease the amount of vitamin D in your blood and might lead to osteoporosis and osteopenia, increasing your risk of breaking a bone. If the amount of vitamin D in your blood is low, your doctor will give you vitamin D supplements
• cause nerve damage (peripheral neuropathy), which can lead to symptoms such as numbness and tingling in hands or feet
• make the skin on your face look and feel coarse and become slightly hairy
• affect your ability to think clearly

Other side effects

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

Phenytoin and pregnancy

Phenytoin has been linked to an increased risk of problems for your baby if you take it during pregnancy.

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

If you take phenytoin for epilepsy, it's important that this is controlled during pregnancy, as seizures can harm you and your baby. If you become pregnant, keep taking phenytoin, but talk to your doctor straight away. It's likely that you will need to be seen in a specialist epilepsy clinic, and they may want to change your medicine.

If you're trying to get pregnant or have become pregnant while taking phenytoin, it's recommended you take a high dose of folic acid (5mg a day). You can get this from your doctor or midwife.

Ideally, it's best to take high dose folic acid for 3 months before you start trying to get pregnant and for the first 12 weeks of pregnancy. Do not worry if you have not taken it before you get pregnant, but start taking it as soon as possible once you know that you are pregnant. It helps your baby to grow normally.

Your baby may need extra monitoring for a few days after they're born. This is because they can sometimes have withdrawal symptoms from phenytoin.

Phenytoin and breastfeeding

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

Phenytoin passes into breast milk in small amounts. Most babies do not get any side effects, but some babies might be more sleepy or not feed as well.

Taking other medicines while you're taking phenytoin might increase the chance of your baby getting these side effects, although this is still rare.

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 is not feeding as well as usual, seems unusually sleepy or irritable, or has a rash, or you have any other concerns about them, talk to your pharmacist, midwife, health visitor or doctor as soon as possible.

Phenytoin and fertility

There's no good evidence to suggest that taking phenytoin causes fertility problems in men or women.

However, speak to your doctor if you're female and trying to get pregnant. They may want to review your treatment.

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 or vigabatrin
• medicines used to treat HIV
• hormonal contraceptives or hormone replacement therapy (HRT)
• medicines used for heart conditions and to help prevent blood clots like digoxin, amiodarone, furosemide, warfarin, diltiazem, nifedipine, apixaban or ticagrelor
• medicines used to treat fungal infections like fluconazole, itraconazole or miconazole
• medicines used for tuberculosis (TB) and other infections like isoniazid, rifampicin, sulfamethoxazole-trimethoprim, doxycycline or ciprofloxacin
• medicines used for stomach problems like omeprazole, sucralfate, ranitidine or some antacids
• theophylline, a medicine to help with breathing problems
• medicines used for sleeping problems (insomnia) and mental health problems like chlordiazepoxide, clozapine, clobazam, diazepam, disufiram, fluoxetine, quetiapine, trazodone or amitriptyline
• medicines used for transplants such as ciclosporin or 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.

There's not enough information to say that other herbal remedies and supplements are safe to take with phenytoin. They're not tested in the same way as pharmacy and prescription medicines.

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