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What the label lists and what reports show

Phenytoin side effects

Phenytoin's FDA label: “The most common adverse reactions encountered with phenytoin therapy are nervous system reactions and are usually dose-related.” FAERS holds 40,278 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 convulsion (3,518 reports).

  • Generic listed

Check it at the source: the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.); the FDA FAERS public dashboard.

What does the patient leaflet say about side effects?

From the patient leaflet: What are the possible side effects of PHENYTEK ® CAPSULES?

See “ What is the most important information I should know about PHENYTEK ® CAPSULES?” PHENYTEK ® CAPSULES may cause other serious side effects including: • Liver problems. • Low blood count which could increase your chance of getting infections, bruising, bleeding and increased fatigue • Softening of your bones (osteopenia, osteoporosis, and osteomalacia) can cause your bones to break (fractures). • High blood sugar (hyperglycemia). • High levels of phenytoin sodium in your blood that could cause confusion also known as delirium, psychosis or a more serious condition that affects how your brain works (encephalopathy). Call your healthcare provider right away, if you have any of the symptoms listed above. […]

Source: FDA drug label, Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), via openFDA; the full label on DailyMed

What does the label list?

From the label: adverse reactions

The most common adverse reactions encountered with phenytoin therapy are nervous system reactions and are usually dose-related.

From the label: adverse reactions, continued

The following serious adverse reactions are described elsewhere in the labeling: • Withdrawal Precipitated Seizure, Status Epilepticus [see Warnings and Precautions (5.1) ] • Suicidal Behavior and Ideation [see Warnings and Precautions (5.2) ] • Serious Dermatologic Reactions [see Warnings and Precautions (5.3) ] • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity [see Warnings and Precautions (5.4) ] • Hypersensitivity [see Warnings and Precautions (5.5) ] • Cardiac Effects [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) ] • Effects on Vitamin D and Bone [see Warnings and Precautions (5.10) ] • Exacerbation of Porphyria [see Warnings and Precautions (5.12) ] • Teratogenicity and Other Harm to the Newborn [see Warnings and Precautions (5.13) ] • Hyperglycemia [see Warnings and Precautions (5.14) ] The following adverse reactions associated with the use of PHENYTEK ® capsules were identified in clinical studies or postmarketing reports. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure. …

Source: FDA drug label, Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), via openFDA; the full label on DailyMed

What do FAERS reports show?

40,278 reports list phenytoin among the medicines taken, each a report of something that happened, not proof the medicine caused it; 4,416 are coded as a death and 14,794 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 20 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)4,500
  • Convulsion3,518
  • Seizure (a use or a product term, not a reaction)3,095
  • Drug Interaction (a use or a product term, not a reaction)2,262
  • Toxicity To Various Agents1,888
  • Off Label Use (a use or a product term, not a reaction)1,682
  • Fall1,602
  • Dizziness (named in the label)1,539
  • Drug Hypersensitivity1,354
  • Headache (named in the label)1,302
  • Fatigue (named in the label)1,289
  • Nausea (named in the label)1,281
  • Stevens-Johnson Syndrome (named in the label)1,225
  • Pyrexia1,158
  • Vomiting (named in the label)1,123
  • Rash (named in the label)1,120
  • Somnolence (named in the label)1,101
  • Confusional State1,039
  • Condition Aggravated (a use or a product term, not a reaction)1,030
  • Status Epilepticus1,006
  • Asthenia929
  • Gait Disturbance879
  • Pain854
  • Epilepsy (named in the label)822
  • Hypotension819

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

What next?

Phenytoin: the full record
label, generics, price, shortages, recalls
Side effects hub
the most reported medicines and reactions