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Fosphenytoin Side Effects

The most commonly reported side effects of fosphenytoin include off label use, seizure, and status epilepticus, based on 1,062 FDA adverse event reports from 2004 to 2025. 11.3% of reports found the drug to be ineffective.

#1 Reported: Off Label UseAll side effects ↓Top Co-Medication: LevetiracetamDrug interactions ↓43% of Reports Are AdultDemographics ↓

Fosphenytoin side effects

Percentages show how often each reaction appears relative to total reports for fosphenytoin.

1
Drug Ineffective11.3%120
2
Off Label Use7.1%75
3
Seizure4.2%45
4
Status Epilepticus4.2%45
5
Convulsion3.4%36
6
Toxicity To Various Agents3.4%36
7
Cardiac Arrest3.4%36
8
Pyrexia2.9%31
9
Drug Ineffective For Unapproved Indication2.4%26
10
Hypotension2.4%25
11
Altered State Of Consciousness2.3%24
12
Blood Pressure Decreased2.3%24
13
Respiratory Depression2.2%23
14
Hypotension2.1%22
15
Hepatic Function Abnormal2.1%22

These are voluntary reports and do not establish that fosphenytoin caused these reactions.

Report severity

86.7%Serious921 reports
37.8%Hospitalizations401 reports
16.3%Fatal173 reports

Seriousness is determined by the reporter, not by OpenClaim.

Fosphenytoin drug interactions

Other drugs that appear in adverse event reports alongside fosphenytoin. Drugs flagged as a suspected cause appear in the first column. Drugs taken at the same time but not suspected appear in the second.

Also suspected

1
Levetiracetam22.8%242
2
Midazolam10.6%113
3
Lacosamide9.6%102
4
Phenytoin9.1%97
5
Phenobarbital-sodium8.0%85
6
Propofol5.1%54
7
Lorazepam4.9%52
8
Valproic-acid3.5%37
9
Topiramate3.5%37
10
Diazepam3.3%35
11
Clonazepam2.8%30
12
Pentobarbital2.4%26
13
Carbamazepine2.3%24
14
Oxcarbazepine2.3%24
15
Clobazam2.3%24

Taken alongside

1
Levetiracetam12.3%131
2
Midazolam10.0%106
3
Diazepam8.7%92
4
Lorazepam7.3%77
5
Propofol6.5%69
6
Acetaminophen6.1%65
7
Furosemide5.0%53
8
Valproic-acid5.0%53
9
Phenobarbital-sodium4.9%52
10
Fentanyl4.7%50
11
Carbamazepine4.7%50
12
Vancomycin4.1%44
13
Phenytoin4.1%44
14
Methylprednisolone3.9%41
15
Lacosamide3.7%39

Co-occurrence in adverse event reports does not establish a drug interaction. Patients often take multiple medications, and these lists reflect prescribing patterns rather than causal relationships. Consult a healthcare provider about potential drug interactions.

Who reports fosphenytoin side effects

42.7% of fosphenytoin adverse event reports involve female patients and 44.6% involve male patients. The largest age group is adult at 43%. These figures reflect who reports side effects, not underlying risk.

Sex

Female42.7%
Male44.6%
Unknown12.6%

Age group

< 211.6%
2–1111.5%
12–174.5%
18–6442.9%
65+29.5%

What is fosphenytoin used for

Conditions and purposes for which patients were taking fosphenytoin when the adverse event was reported.

Acute Encephalitis With Refractory, Repetitive Partial SeizuresAlcoholic SeizureAnaplastic OligodendrogliomaAnticonvulsant Drug Level AbnormalAnticonvulsant Drug Level Below TherapeuticAtrial FibrillationAcute Encephalitis With Refractory, Repetitive Partial SeizuresBrain ContusionBrain OedemaCardio-respiratory ArrestCerebellar HaemorrhageCerebrovascular AccidentCerebrovascular Accident ProphylaxisChronic Obstructive Airways DiseaseClonic Convulsion

Showing 15 of 89 indications

Fosphenytoin brand names and reporting trend

Fosphenytoin is sold under the brand name Fosphenytoin Sodium.

Brand names

Fosphenytoin Sodium1,244

Quarterly reports (2004–2025)

20052010201520202025

An upward trend reflects increased FAERS reporting overall, not necessarily increased risk.

Taking fosphenytoin with another medication?

Ask OpenClaim about your specific drug combination. Get a sourced report using FDA data, drug labels, and medical literature.

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