Fosphenytoin Side Effects
Some side effects of fosphenytoin may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.
For the Consumer
Applies to fosphenytoin: injectable solution
Get emergency medical help if you have any of these signs of an allergic reaction while taking fosphenytoin: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Tell your caregivers at once if you have a serious side effect such as:
severe burning, itching, swelling, redness, or skin discoloration anywhere in the body;
problems with vision or speech;
feeling like you might pass out;
chest pain, irregular heart rhythm, feeling short of breath;
fever, swollen glands, body aches, flu symptoms;
skin rash, easy bruising or bleeding, severe tingling, numbness, pain, muscle weakness;
confusion, nausea and vomiting, swelling, rapid weight gain, urinating less than usual or not at all;
new or worsening cough with fever, trouble breathing;
upper stomach pain, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes);
low potassium (confusion, uneven heart rate, extreme thirst, increased urination, leg discomfort, muscle weakness or limp feeling); or
severe skin reaction -- fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain, followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling.
Less serious side effects of fosphenytoin may include:
constipation, mild nausea, dry mouth;
headache, dizziness, drowsiness;
mild itching or tingly feeling;
tremor, muscle weakness, loss of coordination;
ringing in your ears; or
pain in your hips or back.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects.
For Healthcare Professionals
Applies to fosphenytoin: injectable solution
In general, cardiovascular collapse and central nervous system depression are the most important adverse sequelae and are more common in debilitated patients, during rapid administration rates, and with higher doses (e.g. >15 mg/kg at 150 mg/min).
The severity and/or frequency of adverse side effects due to fosphenytoin (and phenytoin, the active metabolite) may be increased in patients with renal and/or hepatic disease or in those with hypoalbuminemia. Fosphenytoin conversion to phenytoin may be increased without a concomitant increase in phenytoin clearance.
Nervous system side effects commonly reported have included transient paresthesias (described as itching, burning or tingling) at a site distant from the injection site (most commonly the groin area). Somnolence, nystagmus, dizziness, stupor, and ataxia (which may be signs of toxicity) have also been reported.
The paresthesia reaction is more common with fosphenytoin than phenytoin and does not appear to be an allergic process. The paresthesias usually begin several minutes within the start of the infusion and resolve within 10 minutes after completion of the infusion. Continuation of the infusion during the paresthesia side effect does not appear to result in permanent adverse effects, although the outcome has not been studied in a controlled fashion.
Other less common nervous system effects have included increased reflexes, speech disorder, dysarthria, intracranial hypertension, and nervousness.
Cardiovascular side effects including hypotension, hypertension, bradycardia, tachycardia, and vasodilation have been reported. A case of hypocalcemia-like ECG changes has also been reported.
Cardiovascular side effects appear to be more common with intravenous (versus intramuscular) administration. Elderly and/or debilitated patients may be at greatest risk for cardiovascular complications.
Gastrointestinal side effects have included nausea, tongue and taste disorder, constipation, dry mouth, vomiting, and gingival hyperplasia.
Transient pruritus has been reported to occur more frequently with higher doses and faster infusion rates.
Local injection-site pain tends to be more common with phenytoin than with fosphenytoin. However, infusion disruptions due to systemic burning, itching and/or paresthesia occur more frequently with fosphenytoin than with phenytoin. Pruritus in the perineum appears to be common. Localized injection site pain and burning result in more infusion discontinuations in patients treated with phenytoin. Intramuscular fosphenytoin is well tolerated, with transient mild to moderate itching being the most common local effect. In general, intravenous infusions of fosphenytoin and phenytoin are completed in 5 to 7 and 15 to 20 minutes, respectively. Shorter infusion times with fosphenytoin may result in less local irritation.
Local side effects have included transient pruritus.
Other side effects have included tinnitus, fevers, and infrequent headaches.
Tinnitus has been reported with higher doses and infusion rates.
Hematologic side effects including ecchymosis have been reported infrequently.
Serious, sometimes fatal hematologic events have been reported with phenytoin. Thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, lymphadenopathy, and pancytopenia with or without bone marrow suppression have occurred with phenytoin use. Fatalities have been reported. In cases of severe blood dyscrasias, selection of an alternative to fosphenytoin therapy may be the most prudent decision.
Lymphadenopathy (local or generalized) has been reported with phenytoin use and while a causal relationship has not been determined, differentiation from other lymph node pathologies should be made. Any patient who develops lymphadenopathy should be observed for an extended period of time and alternative seizure control medications should be instituted if at all possible.
Slow metabolism (due to a genetic lack of inducing enzyme) has occurred rarely in patients treated with phenytoin.
Metabolic side effects such as hypokalemia may occur.
Musculoskeletal side effects have included myasthenia.
Pruritus with fosphenytoin is a dose related reaction which has been reported to develop more frequently if the drug is administered intravenously. The groin is the area affected most frequently. The pruritus may last for hours.
Dermatologic side effects including pruritus (up to 49% in one major clinical trial), bullous rash, exfoliative dermatitis, gingival hyperplasia, and erythema multiforme have been reported.
Hepatic side effects including acute hepatotoxicity have been reported with the use of phenytoin. Therefore, if hepatotoxicity occurs with the use of fosphenytoin, the drug should be discontinued and not readministered.
More fosphenytoin resources
- fosphenytoin MedFacts Consumer Leaflet (Wolters Kluwer)
- fosphenytoin Concise Consumer Information (Cerner Multum)
- fosphenytoin Injection Advanced Consumer (Micromedex) - Includes Dosage Information
- Fosphenytoin Prescribing Information (FDA)
- Fosphenytoin Professional Patient Advice (Wolters Kluwer)
- Cerebyx Monograph (AHFS DI)
- Cerebyx Prescribing Information (FDA)
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