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).

This medicine may be used to prevent or treat seizures during or after brain surgeries.
How do I use this medicine?
Phenytoin injection is to be given intravenously (into the vein) as a slow injection or infusion, or intramuscularly (into the muscle) as an injection.

Your doctor or nurse will administer the injection for you.

The dose of this medicine will be decided by your doctor. Your doctor will advise you on the treatment timeframe depending on your condition and response to the medication.
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:
  • history of liver problems due to Phenytoin
  • heart rhythm problems e.g. slow heartbeat, heart block
as Phenytoin may not be suitable for you.

Do not use Phenytoin with delavirdine (medicine used to treat HIV infection).
What should I take note of while using this medicine?
Inform your doctor if you have the following conditions:
  • low blood pressure
  • diabetes
  • underactive thyroid gland
  • high bilirubin levels in the blood
  • porphyria (an inherited disorder that causes skin or nervous system abnormalities)
  • muscle weakness disorder
  • liver disease
  • kidney disease
Let your doctor know if you are pregnant. Phenytoin may cause harm to your unborn child.

It is important that you do not get pregnant while using this medicine. You must use proven birth control methods during Phenytoin therapy. Birth control medicines containing hormones may not be effective. You may wish to discuss other reliable methods of birth control with your doctor.

Inform your doctor if you are breastfeeding.

Be sure to check with your doctor or pharmacist before giving this medicine to a child, the elderly, or a debilitated individual (physically or mentally weak, usually due to illness or old age). Children, elderly people, and debilitated individuals may be more sensitive to the side effects.

Some genetic traits can cause serious allergic reactions (e.g. rashes with peeling of the skin or blisters in the lips, mouth, or eyes) from this medicine. If you have a certain genetic variant known as HLA-B*15:02, you are more likely to develop these reactions to Phenytoin. Moreover, individuals who are intermediate or poor metabolisers of CYP2C9 may experience enhanced side effects of this medicine (e.g. sleepiness, slurred speech). Your doctor may perform genetic testing before giving you this medicine to know if it is suitable for you.

If you are of a Black race, you may be at risk of developing certain side effects (e.g. liver problems, serious allergic or skin reactions). Inform your doctor if you are using this medicine.

If you are going to have certain laboratory tests (e.g. thyroid function tests, blood sugar levels), inform your doctor that you are using Phenytoin.

Why is it important to keep my appointments with the doctor?

Keep your appointments with your doctor. Your doctor needs to monitor your condition and check your response to the medication regularly.
  • Pregnancy test may be done before treatment to know if this medicine is suited for you to use.
  • Routine tests (e.g. liver function, complete blood count, Phenytoin levels in the blood) may be done while you are being treated with this medicine. Your doctor will advise you about how often you need to have these tests.
  • Regular monitoring of your blood pressure and for signs of skin or allergic reactions and suicidal thoughts and behaviour may also be needed.
What side effects could I experience?
Phenytoin may cause drowsiness or dizziness. If affected, do not drive or take part in any activity in which you need to be alert.

This medicine 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: headache, nausea, vomiting, constipation, changes in taste, difficulty sleeping, and injection site pain or irritation.

Some side effects may need immediate medical help. Alert your doctor quickly if you experience any of the following:
  • rashes, breathlessness, swelling of the face, eyes, mouth, or throat
  • slow or irregular heartbeat, chest pain
  • yellowing of the skin or eyes, abdominal pain, dark coloured urine, tiredness, swelling in the legs and ankles
  • bone pain or fractures
  • irregular eye movements, slurred speech, trouble controlling body movements
  • unusual changes in mood or behaviour (being too depressed or agitated) or having thoughts of self-harm
  • confusion or hallucinations (seeing, hearing, or feeling things that are not existent)
  • rashes with peeling of the skin or blistering of the lips, mouth, or eyes accompanied by fever
Phenytoin may cause the level of your red blood cells, white blood cells, and platelets to drop.

Red blood cells carry oxygen around your body. A fall in the level of red blood cells may make you feel tired and worn out.

White blood cells help your body to fight infections. A fall in the level of your white blood cells may put you at higher risk for infections, such as coughs, colds, and flu, which may lead to more serious infections. Avoid crowded places and people who are sick. Inform your doctor if you have a fever, cough, or flu that does not go away.

Platelets help your blood to clot when there is a cut in the skin. A fall in the level of your platelets may put you at risk of bleeding more than usual. Do not take part in activities where you may fall or get injured, such as contact sports. Inform your doctor if you get any unusual bruising (large bruises or several bruises, especially if the bruises appeared on their own) or bleeding that takes a long time to stop (for example, too much bleeding when you floss or brush your teeth).

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 delavirdine (medicine used to treat HIV infection).

Inform your doctor and pharmacist if you are taking or using any of these medicines:
  • other medicines for epilepsy (fits or seizures) e.g. carbamazepine, valproic acid, lamotrigine, phenobarbital
  • medicines for irregular heartbeat e.g. amiodarone, digoxin, mexiletine, verapamil
  • medicines to treat fungal infections e.g. fluconazole, itraconazole, ketoconazole
  • medicines for cancer e.g. fluorouracil, doxorubicin, bleomycin
  • blood-thinning medicines e.g. warfarin, apixaban, ticagrelor
  • cholesterol-lowering medicines e.g. atorvastatin, simvastatin
  • certain antibiotics e.g. chloramphenicol, erythromycin, sulfamethoxazole-trimethoprim, doxycycline, ciprofloxacin
  • medicines for depression e.g. paroxetine, fluoxetine, fluvoxamine
  • medicines to treat TB (lung infection known as tuberculosis) e.g. isoniazid, rifampicin
  • medicines for high blood pressure e.g. nifedipine, reserpine
  • medicines for HIV infection e.g. ritonavir, efavirenz, fosamprenavir
  • theophylline (asthma medicine)
  • tacrolimus (medicine used in organ transplants or certain immune disorders)
  • medicines that reduce stomach acid production e.g. cimetidine, omeprazole
  • tolbutamide (medicine for diabetes)
  • St. John's wort (herbal medicine)
This list does not include all medicines that may interact with Phenytoin.

Inform your doctor if you are currently taking birth control pills as this medicine may reduce the effectiveness of hormonal contraceptives. You should not stop or start any birth control pills without first informing your doctor. Ask your doctor about using non-hormonal birth control as an alternative while being treated with Phenytoin.

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 in a cool, dry place away from the reach of children.

Do not refrigerate the diluted infusion mixture otherwise, it will no longer be effective and should not be used.

If you notice that the injection has become cloudy, do not use it. Throw it away and use a new 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 © 2022 MIMS. All rights reserved. Powered by
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