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:
Headaches
Make sure you rest and drink plenty of fluids. Do not drink too much alcohol. 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 better.
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 (nausea or vomiting)
Stick to simple meals and do not eat rich or spicy food. It might help to take your phenytoin after a meal or snack. Drink plenty of fluids such as water or squash. 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.
If you take the combined contraceptive pill or progestogen-only pill and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Constipation
Eat more high-fibre foods, such as fresh fruit and vegetables, and cereals. Drink several glasses of water or another non-alcoholic liquid every 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.
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.
Immediate action required: Go to A&E now if:
- you get a severe rash with flushing, blisters or ulcers – these can be signs of Stevens-Johnson syndrome
Other serious side effects
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
Immediate action required: Call 999 or go to A&E now if:
- you have thoughts of harming or killing yourself
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to phenytoin.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Long-term side effects
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.
You can report any suspected side effect using the Yellow Card safety scheme.