Phenytoin - oral

Patient Medicine Information
Why do I need this medicine?
Phenytoin is used on its own or together with other medicines to treat epilepsy (also known as fits or seizures).

It may be used to prevent or treat seizures during or after certain brain surgeries.

Phenytoin also helps to relieve pain in a painful nerve condition that affects the face called trigeminal neuralgia.
How do I take this medicine?
Take Phenytoin exactly as directed by your doctor or according to the instructions on the label. Do not take more or less than instructed by your doctor.

You may take this medicine with food to reduce any stomach discomfort that it may cause. Try to take it at the same time each day.

If you are taking the tablet, capsule, or extended-release type of tablet (usually labelled as “ER”), swallow it whole with a glass of water. Do not divide, open, chew or crush it.

If you are taking the chewable tablet, it should be chewed thoroughly before swallowing.

If you are taking the oral suspension, shake the bottle well before you take it to ensure that the liquid is evenly mixed. Use the measuring spoon or cup provided to measure out your dose.

If you are on nasogastric feeding or enteral feeds, take your dose of Phenytoin at least 2 hours before or after feeds. Always flush the tubing with water after each dose of Phenytoin to ensure that the full dose is taken.

Phenytoin must be taken regularly for it to be effective. Continue taking Phenytoin even when you feel better. Phenytoin is not a cure and you may need to take it long-term to keep your seizures under control. Do not stop taking Phenytoin suddenly as stopping it may worsen your condition.
What should I do if I have forgotten to take this medicine?
Take the missed dose as soon as you remember. If it is almost time for your next dose, skip the missed dose and return to your normal dosing schedule.

DO NOT double a dose under any circumstances.

If you often forget to take your medicine, let your doctor and pharmacist know.
When should I not use this medicine?

Alert your doctor if you have the following conditions:

  • heart disease such as irregular heartbeat
  • history of liver disease due to Phenytoin

as Phenytoin may not be suitable for you.

Alert your doctor immediately if you ever had an allergic reaction to Phenytoin or other similar medicines such as carbamazepine.

Do not take Phenytoin with medicines used to treat HIV infection, such as delavirdine.

What should I take note of while taking this medicine?

Inform your doctor if you have the following conditions:

  • liver disease
  • kidney disease
  • blood disorders
  • diabetes
  • osteoporosis or bone development problems
  • porphyria (an inherited disorder that causes skin or nervous system abnormalities)
  • hypothyroidism (low level of thyroid hormone due to decreased activity of thyroid gland)

Let your doctor know if you are pregnant or breastfeeding. Discuss the use of appropriate birth control methods with your doctor while being treated with Phenytoin.

If you are going to have an operation, including minor surgery and dental work, inform your doctor or dentist that you are taking this medicine.

Some genetic traits (HLA-B*15:02) can cause serious skin reactions (e.g. potentially life-threatening skin rashes) from this medicine. Your doctor may need to perform genetic testing to know if this medicine is suitable for you.

Keep your appointments with your doctor. Your doctor needs to monitor your condition and check your response to the medication regularly. For as long as you are taking this medicine, you may need to have regular blood tests to check your body’s response to the medicine. Your doctor will advise you about how often you need to have blood tests.

What side effects could I experience?

Phenytoin may cause any of the following side effects: dizziness, tiredness and drowsiness. If affected, do not drive or take part in any activity in which you need to be alert or see clearly.

Phenytoin may also cause swelling and soreness of the gums, especially in children. To minimise gum swelling, observe good oral hygiene by regularly brushing and flossing your teeth.

Other side effects include any of the following: nausea, vomiting, constipation, loss of appetite, difficulty sleeping, double or blurred vision, headache, unusual eye movements and uncoordinated movements. These side effects are common especially when you have just started taking Phenytoin.

Some side effects may need immediate medical help. Alert your doctor quickly if you experience any of the following:

  • rashes with skin peeling or blistering of the lips, mouth or eyes accompanied by fever
  • breathlessness, swelling of the face, eyes or mouth
  • frequent and persistent sore throat with fever
  • unusual bleeding or bruising
  • yellowing of the skin or eyes with abdominal pain, dark coloured urine, tiredness, swelling in the legs and ankles
  • joint pain
  • confusion or hallucinations (seeing, hearing or feeling things that are not existent)
  • unusual changes in mood or behaviour (being too depressed or agitated, or having compulsive and impulsive behaviour)
  • having thoughts of self-harm

Inform your doctor if any of these side effects do not go away or are severe, or if you experience other side effects.

Can I take this with other medicines?
DO NOT take Phenytoin with medicines used to treat HIV infection, such as delavirdine.

Inform your doctor and pharmacist if you are taking any of these medicines:

  • medicines for sleep disorder or anxiety e.g. diazepam, chlordiazepoxide
  • medicines for irregular heartbeat e.g. amiodarone, quinidine
  • medicines to treat cancer e.g. fluorouracil
  • other medicines to treat epilepsy (fits) e.g. valproic acid, felbamate
  • medicines that reduce stomach acid production e.g. cimetidine, omeprazole
  • tolbutamide (medicine for diabetes)
  • medicine for pain and inflammation e.g. phenylbutazone
  • blood-thinning medicines e.g. warfarin
  • asthma medicines e.g. theophylline
  • birth control pills e.g. etonogestrel, levonorgestrel
  • medicines to treat fungal infections e.g. fluconazole, itraconazole, ketoconazole
  • certain antibiotics e.g. chloramphenicol, ciprofloxacin
  • medicines used in organ transplant or certain immune disorders e.g. tacrolimus
  • medicines to treat TB (lung infection known as tuberculosis) e.g. isoniazid, rifampicin
  • medicine used to treat alcoholism e.g. disulfiram
  • St John's wort (herbal medicine)

This list does not include all medicines that may interact with Phenytoin.

As Phenytoin may reduce the effectiveness of birth control pills, ask your doctor about the use of additional birth control methods.

Always notify your doctor and pharmacist if you are taking any other medicines, including herbal tonics such as traditional Chinese medicines, supplements and medicines that you buy without a prescription.

What special dietary instructions should I follow?
Avoid alcohol.
How should I store this medicine?
Store below 30°C for tablets and capsules. Store below 25°C for suspension. Do not freeze, otherwise, this medicine will become ineffective and should not be used.

Store away from the reach of children.

Protect from light and moisture.

Medicines must not be used past the expiry date.
This information is independently developed by MIMS based on phenytoin - oral and is provided for your reference only. It is not a replacement for and should only be used in conjunction with full consultation with a licensed healthcare professional, the information provided by your pharmacist and/or the manufacturer of the medication. It may not contain all the available information you require and cannot substitute professional medical care, nor does it take into account all individual circumstances. Although great effort has been made to ensure content accuracy, we shall not be held responsible or liable for any claims or damages arising from the use or misuse of the information contained herein, its contents or omissions, or otherwise. Copyright © 2022 MIMS. All rights reserved. Powered by
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