Common questions about phenytoin

For epilepsy, brain cells normally talk to each other using electrical signals and chemicals. Seizures are bursts of electrical activity in the brain that can temporarily affect how it works.

Phenytoin slows down these electrical signals to stop seizures.

For nerve pain, phenytoin was not originally designed to treat pain, but it can help to relieve nerve pain, such as trigeminal neuralgia, by slowing down electrical impulses in the nerves and reducing their ability to transmit pain.

It usually takes around 4 weeks for phenytoin to work fully.

This is because the dose of phenytoin needs to be increased slowly to prevent side effects.

You may still have seizures or pain during this time.

Many people can take phenytoin safely for several months or years.

But there are some side effects that might happen over a long time.

Long-term treatment with phenytoin 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

If you have epilepsy, it's best to stay on the same brand of phenytoin, as other brands of phenytoin may not work as well for you.

If you take phenytoin for trigeminal neuralgia, most people do not have to stay on the same brand.

Talk to your doctor or pharmacist if you cannot get a supply of your normal brand of phenytoin.

If you're switching medicines, it's very important to follow your doctor's advice.

If you want to switch, you'll usually start taking the new one at a low dose and slowly build up the dose while you're still taking phenytoin.

Once you're taking the correct dose of the new medicine, you should be able to slowly reduce your dose of phenytoin.

It can take several weeks or months until you have stopped taking phenytoin completely.

There are lots of other medicines that can be used for epilepsy, but they work in different ways.

They might have different side effects, or be taken more or less often.

Your doctor will discuss the best medicine for you. The choice will depend on the type of seizures that you have and how often you have them.

It's not possible to say that one works better than the other. It varies from person to person.

Other epilepsy medicines include:

• brivaracetam
• carbamazepine
• clobazam
• ethosuximide
• gabapentin
• levetiracetam
• lamotrigine
• pregabalin
• sodium valproate
• topiramate

Before prescribing a medicine, your doctor will also take into account your age and sex, the medicines you're already taking and any other health problems you have.

Your doctor will try to treat epilepsy using a single medicine. If this medicine is not working well or you're getting side effects, your doctor will usually try you on a different one.

If a single medicine is not preventing your seizures, your doctor may recommend taking 2 or more epilepsy medicines at the same time.

There are lots of other medicines that can be used for trigeminal neuralgia, but they work in different ways.

An epilepsy medicine called carbamazepine is usually the first medicine given to treat trigeminal neuralgia.

Phenytoin might be used if carbamazepine does not work or causes too many side effects.

Other medicines that your doctor might use include gabapentin.

Your doctor or specialist will find the medicine that works best for you.

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

Coming off 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.

Your doctor may measure the amount of phenytoin in your blood:

• when you first start taking it
• when you change your dose
• if you need to change to a different form of phenytoin (for example, if you change from tablets to the liquid)

The blood tests will help your doctor to decide what dose is right for you and if they need to increase or decrease your dose.

You can take phenytoin as normal on the day of your surgery, unless your doctor or surgeon has advised you not to.

If you have epilepsy, you're entitled to free prescriptions for all of your medicines, not just your epilepsy ones.

To claim your free prescriptions, you'll need a medical exemption certificate.

The application form for the medical exemption certificate is called FP92A. You can get this from your doctor's surgery. You'll need to fill in the form, and your doctor will sign it and send it off.

You can use the contraceptive injection or coil safely with phenytoin.

However, phenytoin might stop these contraceptives working:

• combined hormonal contraceptives (the combined pill, patches and vaginal ring) – look out for bleeding between periods, which might be a sign the pill is not working. Talk to your doctor if this happens. Your doctor may advise you to use a different type of contraceptive, or condoms as well as combined hormonal contraception
• progestogen-only contraceptive pill and implants – your doctor may advise you to use a different type of contraceptive that's not affected by phenytoin
• emergency contraception – tell your doctor or pharmacist that you take phenytoin if you need emergency contraception. You may need an increased dose or to use the copper coil

If phenytoin is not working for you and you're being sick, your contraceptive pills may not protect you from pregnancy. Check the pill packet to find out what to do.

Find out more:

• What to do if you're sick or have diarrhoea when taking the combined pill
• What to do if you're sick or have diarrhoea when taking the progestogen-only pill

You may feel sleepy or dizzy when you first start taking phenytoin or the dose is increased.

If this happens to you, do not drive, ride a bike or operate machinery until you feel better.

Phenytoin can also affect your vision. Do not drive or ride a bike if your vision is affected.

It's an offence to drive a car if your ability to drive safely is affected. It's your responsibility to decide if it's safe to drive. If you're in any doubt, do not drive.

Talk to your doctor or pharmacist if you're unsure whether it's safe for you to drive while taking phenytoin. GOV.UK has more information on the law on drugs and driving.

If you have epilepsy, you're not allowed to drive until you have had no seizures for 1 year.

If you change your epilepsy medicine, your doctor will tell you whether you need to stop driving and for how long.

Yes, you can usually drink alcohol with phenytoin.

But it may make you feel sleepy or tired, and alcohol and hangovers can bring on seizures in some people with epilepsy.

During the first few days of taking phenytoin, it's best to stop drinking alcohol until you see how the medicine affects you.

Talk to your doctor about how much alcohol you drink. They may do blood tests to check everything is OK.

No, you can eat and drink normally while taking phenytoin.