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Fosphenytoin Injection

pronounced as (fos fen' i toyn)


You may experience serious or life-threatening low blood pressure or irregular heart rhythms while you are receiving fosphenytoin injection or afterwards. Tell your doctor if you have or have ever had irregular heart rhythms or heart block (condition in which electrical signals are not passed normally from the upper chambers of the heart to the lower chambers). Your doctor may not want you to receive fosphenytoin injection. Also, tell your doctor if you have or have ever had heart failure or low blood pressure. If you experience any of the following symptoms, tell your doctor immediately: dizziness, tiredness, irregular heartbeat, or chest pain.

You will receive each dose of fosphenytoin injection in a medical facility, and a doctor or nurse will monitor you carefully while you are receiving the medication and for about 10 to 20 minutes afterwards.

Why is this medication prescribed?

Fosphenytoin injection is used to treat primary generalized tonic-clonic seizures (formerly known as a grand mal seizure; seizure that involves the entire body) and to treat and prevent seizures that may begin during or after surgery to the brain or nervous system. Fosphenytoin injection may also be used to control certain type of seizures in people who cannot take oral phenytoin. Fosphenytoin is in a class of medications called anticonvulsants. It works by decreasing abnormal electrical activity in the brain.

How should this medicine be used?

Fosphenytoin injection comes as a solution (liquid) to be injected intravenously (into a vein) or intramuscularly (into a muscle) by a doctor or nurse in a medical facility. When fosphenytoin is injected intravenously, it is usually injected slowly. How often you receive fosphenytoin injection and the length of your treatment depends on how your body responds to the medication. Your doctor will tell you how often you will receive fosphenytoin injection.

Ask your pharmacist or doctor for a copy of the manufacturer's information for the patient.

Other uses for this medicine

This medication may be prescribed for other uses; ask your doctor or pharmacist for more information.

What special precautions should I follow?

Before receiving fosphenytoin injection,

  • tell your doctor and pharmacist if you are allergic to fosphenytoin, other hydantoin medications such as ethotoin (Peganone) or phenytoin (Dilantin, Phenytek), any other medications, or any of the ingredients in fosphenytoin injection. Ask your pharmacist for a list of the ingredients.
  • tell your doctor if you are taking delavirdine (Rescriptor). Your doctor will probably not want you to receive fosphenytoin injection if you are taking this medication.
  • tell your doctor and pharmacist what prescription and nonprescription medications, vitamins, nutritional supplements, and herbal products you are taking or plan to take. Be sure to mention any of the following: albendazole (Albenza); amiodarone (Nexterone, Pacerone); anticoagulants ('blood thinners') such as warfarin (Coumadin, Jantoven); antifungal medications such as fluconazole (Diflucan), ketoconazole (Nizoral), itraconazole (Onmel, Sporanox, Tolsura), miconazole (Oravig), posaconazole (Noxafil), and voriconazole (Vfend); certain antivirals such as efavirenz (Sustiva, in Atripla), indinavir (Crixivan), lopinavir (in Kaletra), nelfinavir (Viracept), ritonavir (Norvir, in Kaletra), and saquinavir (Invirase); bleomycin; capecitabine (Xeloda); carboplatin; chloramphenicol; chlordiazepoxide (Librium, in Librax); cholesterol medications such as atorvastatin (Lipitor, in Caduet), fluvastatin (Lescol), and simvastatin (Zocor, in Vytorin); cisplatin; clozapine (Fazaclo, Versacloz); cyclosporine (Gengraf, Neoral, Sandimmune); diazepam (Valium); diazoxide (Proglycem); digoxin (Lanoxin); disopyramide (Norpace); disulfiram (Antabuse); doxorubicin (Doxil); doxycycline (Acticlate, Doryx, Monodox, Oracea, Vibramycin); fluorouracil; fluoxetine (Prozac, Sarafem, in Symbyax, others); fluvoxamine (Luvox); folic acid; fosamprenavir (Lexiva); furosemide (Lasix); H2 antagonists such as cimetidine (Tagamet), famotidine (Pepcid), nizatidine (Axid), and ranitidine (Zantac); hormonal contraceptives (birth control pills, patches, rings, or injections); hormone replacement therapy (HRT); irinotecan (Camptosar); isoniazid (Laniazid, in Rifamate, in Rifater); medications for mental illness and nausea; other medications for seizures such as carbamazepine (Carbatrol, Equetro, Tegretol, others), ethosuximide (Zarontin), felbamate (Felbatol), lamotrigine (Lamictal), methsuximide (Celontin), oxcarbazepine (Trilepta, Oxtellar XR), phenobarbital, topiramate (Topamax), and valproic acid (Depakene); methadone (Dolophine, Methadose); methotrexate (Otrexup, Rasuvo, Trexall, Xatmep); methylphenidate (Daytrana, Concerta, Metadate, Ritalin); mexiletine; nifedipine (Adalat, Procardia), nimodiwashpine (Nymalize), nisoldipine (Sular); omeprazole (Prilosec); oral steroids such as dexamethasone, methylprednisolone (Medrol), prednisolone, and prednisone (Rayos); paclitaxel (Abraxane, Taxol); paroxetine (Paxil, Pexeva); praziquantel (Biltricide); quetiapine (Seroquel); quinidine (in Nuedexta); reserpine; rifampin (Rifadin, Rimactane, in Rifamate, in Rifater); salicylate pain relievers such as aspirin, choline magnesium trisalicylate, choline salicylate, diflunisal, magnesium salicylate (Doan's, others), and salsalate; sertraline (Zoloft); sulfa antibiotics; teniposide; theophylline (Elixophyllin, Theo-24, Theochron); ticlopidine; tolbutamide; trazodone; verapamil (Calan, Verelan, in Tarka); vigabatrin (Sabril); and vitamin D. Your doctor may need to change the doses of your medications or monitor you more carefully for side effects.
  • tell your doctor what herbal products you are taking, especially St. John's wort.
  • tell your doctor if you have ever developed a liver problem while receiving fosphenytoin injection or phenytoin. Your doctor will probably not want you to receive fosphenytoin injection.
  • tell your doctor if you drink or have ever drunk large amounts of alcohol. Tell your doctor if you have had laboratory testing that reported you have an inherited risk factor that makes it more likely that you could have an serious skin reaction to fosphenytoin. Also, tell your doctor if you have or have ever had diabetes, porphyria (condition in which certain natural substances build up in the body and may cause stomach pain, changes in thinking or behavior, or other symptoms), low levels of albumin in your blood, or kidney or liver disease.
  • tell your doctor if you are pregnant, plan to become pregnant, or are breastfeeding. Talk to your doctor about effective birth control methods that you can use during your treatment. Fosphenytoin may harm the fetus.
  • if you are having surgery, including dental surgery, tell the doctor or dentist that you are receiving fosphenytoin injection.
  • talk to your doctor about the safe use of alcohol while you are taking this medication.
  • talk to your doctor about the best way to care for your teeth, gums, and mouth during your treatment with fosphenytoin injection. It is very important that you care for your mouth properly to decrease the risk of gum damage caused by fosphenytoin.

What side effects can this medication cause?

Fophenytoin may cause an increase in your blood sugar. Talk to your doctor about the symptoms of high blood sugar and what to do if you experience these symptoms.

Fosphenytoin injection may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away:

  • itching, burning, or tingling sensation
  • uncontrollable eye movements
  • abnormal body movements
  • loss of coordination
  • confusion
  • dizziness
  • weakness
  • agitation
  • slurred speech
  • dry mouth
  • headache
  • changes in your sense of taste
  • vision problems
  • ringing of the ears or difficulty hearing
  • constipation

Some side effects can be serious. If you experience any of the following symptoms or those listed in the SPECIAL PRECAUTIONS section, call your doctor immediately:

  • swelling, discoloration, or pain at the injection site
  • blisters
  • rash
  • hives
  • swelling of the eyes, face, throat, or tongue
  • difficulty breathing or swallowing
  • hoarseness
  • swollen glands
  • nausea
  • vomiting
  • yellowing of the skin or eyes
  • pain in upper right part of the stomach
  • excessive tiredness
  • unusual bruising or bleeding
  • small red or purple spots on skin
  • loss of appetite
  • flu-like symptoms
  • fever, sore throat, rash, mouth ulcers, or easy bruising, or facial swelling
  • swelling of the arms, hands, ankles, or lower legs

Fosphenytoin injection may cause other side effects. Call your doctor if you have any unusual problems while you are taking this medication.

If you experience a serious side effect, you or your doctor may send a report to the Food and Drug Administration's (FDA) MedWatch Adverse Event Reporting program online ( or by phone (1-800-332-1088).

Receiving fosphenytoin may increase the risk that you will develop problems with your lymph nodes including Hodgkin's disease (cancer that begins in the lymph system). Talk to your doctor about the risks of using this medication to treat your condition.

In case of emergency/overdose

In case of overdose, call the poison control helpline at 1-800-222-1222. Information is also available online at If the victim has collapsed, had a seizure, has trouble breathing, or can't be awakened, immediately call emergency services at 911.

Symptoms of overdose may include:

  • nausea
  • vomiting
  • fatigue
  • fainting
  • irregular heartbeat
  • uncontrollable eye movements
  • loss of coordination
  • slow or slurred speech
  • uncontrollable shaking of a part of the body

What other information should I know?

Keep all appointments with your doctor and the laboratory. Your doctor may order certain lab tests to check your response to fosphenytoin injection.

Before having any laboratory test, tell your doctor and the laboratory personnel that you are receiving fosphenytoin injection.

It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.

Brand names

  • Cerebyx®
Last Revised - 12/15/2019