Phenytoin: Side Effects, Uses & Monitoring
Fosphenytoin sodium injection, USP is indicated for the control of generalized tonic-clonic status epilepticus and prevention and treatment of seizures occurring during neurosurgery.
- Data source
- FDA labeling via NIH DailyMed (label 87ff8a91…)
- Label version
- current
- Last reviewed
- 2026
Reported adverse reactions
Reactions listed in current FDA labeling, with the highest frequency reported across summarized placebo-controlled trials.
| Adverse reaction (as reported in labeling) | Reported frequency* |
|---|---|
| Nystagmus | 59.1% |
| Pruritus | 48.9% |
| Dizziness | 31.1% |
| Somnolence | 27.3% |
| Ataxia | 18.2% |
| Nausea | 13.6% |
| Hypotension | 9.1% |
| Vomiting | 9.1% |
| Tremor | 9.1% |
| Hypesthesia | 9.1% |
| Tinnitus | 9.1% |
| Amblyopia | 9.1% |
| Stupor | 7.7% |
| Ecchymosis | 7.3% |
| Accidental injury | 6.6% |
| Vasodilatation | 5.6% |
| Reflexes decreased | 4.9% |
| Headache | 4.5% |
| Dry mouth | 4.5% |
| Incoordination | 4.5% |
*Highest percentage reported for the active moiety across placebo-controlled trial tables in current FDA labeling. Values reflect trial reporting, not your individual risk.
Labeled uses (from FDA prescribing information)
Fosphenytoin sodium injection, USP is indicated for the control of generalized tonic-clonic status epilepticus and prevention and treatment of seizures occurring during neurosurgery. Fosphenytoin can also be substituted, short-term, for oral phenytoin. Fosphenytoin should be used only when oral phenytoin administration is not possible. Fosphenytoin must not be given orally.
Tests providers may use to monitor Phenytoin therapy
Frequently Asked Questions
What is Phenytoin used for?
FDA labeling lists the approved uses shown above under "Labeled uses." This summary is informational; only a qualified prescriber can determine whether a medication is appropriate for you.
What are the most commonly reported side effects of Phenytoin?
The table above shows adverse reactions and the highest frequencies reported in placebo-controlled trials summarized in current FDA labeling for Phenytoin. Individual risk varies.
Does Phenytoin require lab monitoring?
Labeling and standard practice associate Phenytoin with periodic laboratory monitoring; the tests above are commonly ordered by clinicians to monitor patients on therapy. Your provider decides what is appropriate.
Where does this information come from?
All clinical facts on this page were extracted from the current FDA-registered product labeling for this ingredient, published by manufacturers through NIH DailyMed. See the source link below.
Medical disclaimer: Educational content extracted from public FDA labeling. Not medical advice. Always consult a qualified healthcare professional before starting, stopping, or changing any medication.