Fosphenytoin - intravenous/intramuscular

Patient Medicine Information
Why do I need this medicine?
Fosphenytoin is used on its own or together with other medicines to treat fits or seizures.

It may be used to prevent or treat seizures that may occur during or after certain brain surgeries and/or head injury.
How do I use this medicine?
Fosphenytoin may be given intravenously (into the vein) or intramuscularly (into the muscle).

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 Fosphenytoin.

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:
  • slow heartbeat or heart block
  • history of liver disease due to Fosphenytoin or phenytoin
  • acute intermittent porphyria (an inherited disorder that may cause skin or nervous system abnormalities)
as Fosphenytoin may not be suitable for you.

Do not use Fosphenytoin with medicines used to treat HIV known as non-nucleoside reverse transcriptase inhibitors, such as delavirdine.
What should I take note of while using this medicine?
Inform your doctor if you have the following conditions:
  • low blood pressure or heart failure
  • stroke
  • liver disease 
  • kidney disease
  • hypothyroidism (low level of thyroid hormone due to decreased activity of thyroid gland)
  • diabetes
  • limitations from eating foods rich in phosphorus
Let your doctor know if you are pregnant or breastfeeding.

It is important that you do not get pregnant while being treated with Fosphenytoin. You may wish to discuss birth control methods with your doctor or pharmacist.

Be sure to check with your doctor or pharmacist before using this medicine to a child or the elderly. Children or elderly people may be more sensitive to the side effects.

Individual's genetic traits are unique from each other. Researchers say that certain genetic traits can affect your response to Fosphenytoin. Individuals who are positive for the HLA-B*15:02 gene may have an increased risk for developing serious skin reactions while those who are intermediate or poor metabolisers of CYP2C9 may have enhanced response to this medicine. Though not routinely done, your doctor may advise you to take genetic testing to check if this drug is best suited for you. If you know you have these gene types, inform your doctor.

Keep your appointments with your doctor. Your doctor needs to monitor your condition and check your response to the medication regularly.
  • Routine tests (e.g. heart, lung and liver function, blood level, phenytoin level) may be done while you are being treated with this medicine.
  • Regular monitoring of blood pressure and skin reactions may also be needed.
Your doctor will advise you about how often you need to have these tests.
What side effects could I experience?
Fosphenytoin may cause drowsiness or dizziness. If affected, do not drive or take part in any activity in which you need to be alert.

Other side effects include any of the following: injection site pain or irritation, nausea, vomiting, headache, ringing in the ears and itchiness (usually in the groin area).

This medicine will 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. Alert your doctor if you have a fever, or a 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. Alert 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).

Some side effects may need immediate medical help. Alert your doctor quickly if you experience any of the following:
  • rashes with peeling of the skin or blistering of the lips, mouth or eyes accompanied by fever
  • rashes, breathlessness, swelling of the face, eyes or mouth
  • yellowing of the skin or eyes, abdominal pain, dark coloured urine, tiredness, swelling in the legs and ankles
  • involuntary eye movements accompanied by uncoordinated movements
  • having thoughts of self-harm
  • confusion or hallucinations (seeing, hearing or feeling things that are not existent)
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 take Fosphenytoin if you are taking medicines used to treat HIV infection known as non-nucleoside reverse transcriptase inhibitors, such as delavirdine.

Inform your doctor and pharmacist if you are taking any of these medicines:
  • medicines for irregular heartbeat e.g. amiodarone, quinidine
  • medicines to treat cancer e.g. doxorubicin, fluorouracil 
  • other medicines to treat epilepsy (fits or seizures) e.g. valproic acid, felbamate
  • other medicines for HIV infection e.g. efavirenz, fosamprenavir
  • medicines for sleep disorder or anxiety e.g. diazepam, chlordiazepoxide
  • medicines to treat depression e.g. fluvoxamine, sertraline 
  • medicines that reduce stomach acid production e.g. cimetidine, omeprazole
  • medicine for diabetes e.g. tolbutamide, glibenclamide
  • medicines to treat fungal infections e.g. fluconazole, itraconazole, ketoconazole
  • medicines for high blood pressure or heart disease e.g. nifedipine, nisoldipine
  • certain antibiotics e.g. chloramphenicol, ciprofloxacin
  • cholesterol-lowering medicines e.g. simvastatin, atorvastatin
  • blood-thinning medicines e.g. warfarin, ticlopidine
  • medicines to treat TB e.g. isoniazid, rifampicin
  • theophylline (medicine for asthma)
  • digoxin (medicine to treat heart disease)
  • tacrolimus (medicine used in organ transplant or certain immune disorders)
  • disulfiram (medicine used to treat alcoholism)
  • vitamin D
  • St. John’s wort (herbal medicine)
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 Fosphenytoin.

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

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 the refrigerator, between 2-8°C. Do not allow to freeze. If frozen, this medicine will become ineffective and should not be used.

Medicines must not be used past the expiry date.
This information is independently developed by MIMS based on fosphenytoin - 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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