Phenytoin

Sounds like 'FEN-eh-toe-in'

Easy-to-read medicine information about phenytoin – what it is, how to take phenytoin safely and possible side effects.

Type of medicine Also called
  • Anti-epileptic medication (to prevent seizures)
  • Dilantin®
  • Dilantin Infatabs® 

What is phenytoin?

Phenytoin is used to treat epilepsy by preventing seizures. It works by stabilising the electrical activity in your brain and in this way reduces seizures. Read more about epilepsy. 

In New Zealand phenytoin is available as:

  • chewable tablets (50 mg)
  • capsules (30 mg and 100 mg)
  • liquid. 

Dose of phenytoin

  • The dose of phenytoin will be different for different people.
  • Your doctor will start you on a low dose, and increase your dose slowly over a few weeks.
  • Phenytoin is usually taken once or two times a day. 
  • Always take your phenytoin exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much phenytoin to take, how often to take it, and any special instructions.

How to take phenytoin

  • Phenytoin is best taken with or after food.
  • Take your doses at the same times each day, usually once or two times a day.
  • For doses that are taken two times a day, take it once in the morning and once in the evening. Ideally, these times are 10–12 hours apart, for example, some time between 7 and 8 am and between 7 and 8 pm.
  • Phenytoin is available as chewable tablets, capsules and liquid.
Formulation How to take it
Tablets
  • Chew the tablets or you can swallow them with a glass of water.
Capsules
  • Swallow the capsules whole with a glass of water, milk or juice.
  • Do not chew the capsules.
Liquid
  • Shake the bottle well before you measure out the dose.
  • Measure the right amount using an oral syringe or medicine spoon. You can get these from your pharmacy.
  • Do not use a kitchen spoon as it will not give you the right amount. Read more: Tips on how to give medicines to babies and children.
  • Limit drinking alcohol while you are taking phenytoin. It can increase your chance of side effects such as dizziness.
  • If you forget to take your dose, take it as soon as you remember. But, if it is nearly time for your next dose, just take the next dose at the right time. Do not take double the dose.
  • Keep taking phenytoin regularly every day (see tips to help you remember to take your medicines regularly). Do not stop taking phenytoin suddenly as this can cause problems; speak to your doctor or nurse before stopping. 

Precautions – before taking phenytoin

  • Are you pregnant, planning a pregnancy or breastfeeding?
  • Do you have liver problems?
  • Are you of Han Chinese or Thai origin?
  • Are you are taking or using any other medicines including birth control tablets? This includes any medicines you are using that are available to buy from a pharmacy, supermarket or natural health store without a prescription.

If so, it’s important that you tell your doctor or pharmacist before you start taking phenytoin. Sometimes a medicine isn’t suitable for a person with certain conditions or it can only be used with extra care.

Possible side effects from taking phenytoin

Like all medicines, phenytoin can cause side effects, although not everyone gets them. Often side effects improve as your body gets used to the new medicine.

Side effects What should I do?
  • Feeling sick (nausea)
  • Stomach upset
  • Try taking phenytoin with food
  • Tell your doctor if troublesome
  • Feeling sleepy, drowsy or tired
  • Problems with your eyesight such as blurred vision or double vision
  • This is common when starting phenytoin and may settle within a week or two.
  • Be careful when driving or using tools until you know how this medicine affects you. Read more epilepsy/seizures and driving 
  • Limit or avoid alcohol.
  • Tell your doctor if troublesome.
  • Headache
  • Unsteady on your feet
  • Tremor
  • Tell your doctor
  • Gum problems such as swollen gums or bleeding gums 
  • Practice good dental hygiene – brush and floss regularly, and visit your dentist regularly.
  • Tell your dentist you are taking phenytoin
  • Changes in mood, such as low mood, feeling down, feeling depressed  
  • Tell your doctor immediately or ring HealthLine 0800 611 116 
  • Signs of an allergic reaction such as skin rash, mouth ulcers, bruising, fever
  • Tell your doctor immediately or ring HealthLine 0800 611 116 
  • Signs of problems with your liver such as severe tummy pain, yellowing of the eyes and skin, dark urine.
  • Tell your doctor immediately or ring HealthLine 0800 611 116 

Interactions

Phenytoin interacts with some other medications and herbal supplements, so check with your doctor or pharmacist before starting phenytoin or before starting any new medicines. Do not take indigestion medication (antacids) within 2 hours of taking phenytoin. Some contraceptives may not work as well while you are taking phenytoin, and for 4 weeks after stopping. Discuss
with your doctor or pharmacist.

Learn more about phenytoin

Phenytoin New Zealand Formulary
Phenytoin (for children) New Zealand Formulary
Dilantin Medsafe Consumer Information Sheet
Epilepsy/seizures and driving NZTA, 2018

References

  1. Phenytoin New Zealand Formulary
Credits: Sandra Ponen, Pharmacist, NZ. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 07 Aug 2019