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What the label lists and what reports show

Fosphenytoin side effects

Fosphenytoin's FDA label: “Most common adverse reactions (incidence ≥10%) are: • Adults: pruritus, nystagmus, dizziness, somnolence, and ataxia • Pediatrics: vomiting, nystagmus, and ataxia ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Pfizer, Inc.” FAERS holds 2,553 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is cardiac arrest (119 reports).

  • Generic listed
  • Boxed warning

Check it at the source: the fosphenytoin FDA label on DailyMed (effective May 18, 2025, Pfizer Laboratories Div Pfizer Inc); the FDA FAERS public dashboard.

What does the label list?

From the label: adverse reactions

Most common adverse reactions (incidence ≥10%) are: • Adults: pruritus, nystagmus, dizziness, somnolence, and ataxia • Pediatrics: vomiting, nystagmus, and ataxia ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Pfizer, Inc.

TABLE 4. Adverse Reaction Incidence Following IV Administration at the Maximum Dose and Rate to Adult Patients with Epilepsy or Neurosurgical Patients (Events in at Least 2% of CEREBYX-Treated Patients)

BODY SYSTEM Adverse EventIV CEREBYX N=90IV Phenytoin The study was not designed to assess comparative safety. N=22
Pelvic Pain40
Asthenia20
Back Pain20
Headache25
Hypotension89
Vasodilatation65
Tachycardia20
Nausea914
Tongue Disorder40
Dry Mouth45
Vomiting29
Nystagmus4459
Dizziness3127
Somnolence2027
Ataxia1118
Stupor85
Incoordination45
Paresthesia40
Extrapyramidal Syndrome40
Tremor39
Agitation30
Hypesthesia29
Dysarthria20
Vertigo20
Brain Edema25

Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.

From the label: adverse reactions, continued

The following serious adverse reactions are described elsewhere in the labeling: • Cardiovascular Risk Associated with Rapid Infusion [see Warnings and Precautions (5.2) ] • Withdrawal Precipitated Seizure, Status Epilepticus [see Warnings and Precautions (5.3) ] • Serious Dermatologic Reactions [see Warnings and Precautions (5.4) ] • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity [see Warnings and Precautions (5.5) ] • Hypersensitivity [see Warnings and Precautions (5.6) ] • Angioedema [see Warnings and Precautions (5.7) ] • Hepatic Injury [see Warnings and Precautions (5.8) ] • Hematopoietic Complications [see Warnings and Precautions (5.9) ] • Sensory Disturbances [see Warnings and Precautions (5.10) ] • Local Toxicity (Including Purple Glove Syndrome) [see Warnings and Precautions (5.11) ] • Exacerbation of Porphyria [see Warnings and Precautions (5.14) ] • Teratogenicity and Other Harm to the Newborn [see Warnings and Precautions (5.15) ] • Hyperglycemia [see Warnings and Precautions (5.16) ] Most common adverse reactions (incidence ≥10%) are: • Adults: pruritus, nystagmus, dizziness, somnolence, and ataxia • Pediatrics: vomiting, nystagmus, and ataxia ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Pfizer, Inc. at 1-800-438-1985 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. …

Source: FDA drug label, NDA020450 (Parke Davis), as published by Pfizer Laboratories Div Pfizer Inc, effective May 18, 2025 (SPL set de5a6711-4abb-4c3d-a18b-266b094ace11), via openFDA; the full label on DailyMed

What do FAERS reports show?

2,553 reports list fosphenytoin among the medicines taken, each a report of something that happened, not proof the medicine caused it; 354 are coded as a death and 1,162 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 16 reactions below, 9 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.

Reports received each year

The 25 most reported reactions

  • Drug Ineffective (a use or a product term, not a reaction)536
  • Off Label Use (a use or a product term, not a reaction)272
  • Status Epilepticus (a use or a product term, not a reaction)169
  • Seizure (a use or a product term, not a reaction)151
  • Cardiac Arrest (named in the label)119
  • Hypotension (named in the label)114
  • Drug Ineffective For Unapproved Indication (a use or a product term, not a reaction)98
  • Multiple-Drug Resistance96
  • Drug Interaction (a use or a product term, not a reaction)93
  • Toxicity To Various Agents84
  • Pyrexia76
  • Stevens-Johnson Syndrome (named in the label)62
  • Convulsion61
  • Bradycardia (named in the label)60
  • Product Use In Unapproved Indication (a use or a product term, not a reaction)54
  • Rash (named in the label)54
  • Treatment Failure (a use or a product term, not a reaction)54
  • Rhabdomyolysis52
  • Respiratory Failure47
  • Drug Reaction With Eosinophilia And Systemic Symptoms (named in the label)46
  • Epilepsy45
  • Overdose (a use or a product term, not a reaction)45
  • Tachycardia (named in the label)45
  • Pneumonia (named in the label)41
  • Thrombocytopenia (named in the label)41

Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms

What next?

Fosphenytoin: the full record
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Side effects hub
the most reported medicines and reactions