Phenytoin - Intravenous/Intramuscular

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 also be used to prevent and treat seizures during or after certain brain surgeries.
How do I use this medicine?
Phenytoin may be given by deep intramuscular (into the muscle) or intravenous (into the vein) injection.

Your doctor or nurse will administer it for you.

The dose of this medicine will be decided by your doctor. Your doctor will advise you on the treatment timeframe depending on the type and severity of your condition.
What should I do if I have forgotten to use this medicine?
Ensure that you keep all appointments with your doctor so that you do not miss any doses. Your doctor also needs to regularly monitor your response to Phenytoin.

If you miss an appointment or miss an injection, alert your doctor or nurse. A replacement appointment or injection should be given as soon as possible.
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 are allergic reaction to Phenytoin or other similar medicines such as carbamazepine.

DO NOT use Phenytoin with medicines used to treat HIV infection, such as delavirdine.

What should I take note of while using 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 using 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. Regular monitoring of blood pressure may be needed. Monitoring of Phenytoin levels in your body may also be needed.
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: injection site irritation, inflammation, swelling and pain; 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 using 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 use this with other medicines?
DO NOT use 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
  • 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 at or below 25°C. Do not freeze this medicine otherwise, it will no longer be effective and should not be used. If you notice the injection has changed colour or become cloudy, do not use it. Throw it away and use a new injection.

Protect from light. Light may cause the medication to lose some of its effectiveness.

Throw away any used portion of the injection.

Medicines must not be used past the expiry date.
This information is independently developed by MIMS based on Phenytoin - Intravenous/Intramuscular 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 © 2021 MIMS. All rights reserved. Powered by
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