phenytoin
Brand names: Dilantin
Phenytoin injection is used to treat certain types of seizures. It can also prevent seizures during or after neurosurgery.
Drug Pricing (NADAC)
Generic Price
$1.61/unit
Generic Available
Yes (13 manufacturers)
Pricing data from NADAC (CMS), effective December 18, 2024. Compare all drug costs →
What it does
This medicine treats generalized tonic-clonic status epilepticus, a type of prolonged seizure.
Common side effects
No common side effects listed.
Key warnings
This medicine can cause serious heart problems if given too quickly.
How It Works
Phenytoin works by slowing down the signals in the brain that cause seizures. It stabilizes nerve cell membranes, reducing excessive electrical activity. This helps to prevent seizures from starting or spreading.
How to Take It
This medicine is given into a vein (IV) by a healthcare provider. The dose depends on your weight and condition. The injection should be given slowly to avoid heart problems. You will be monitored closely during and after the injection.
Pregnancy & Breastfeeding
Taking phenytoin during pregnancy may increase the risk of birth defects. If you are pregnant or plan to become pregnant, talk to your doctor about the risks and benefits of this medicine.
Missed Dose
Since this medicine is given by a healthcare provider, you are not likely to miss a dose.
Storage
This medicine should be stored at room temperature, away from heat and light.
Side Effects (from patient reports)
Based on 12,150 FDA adverse event reports.
FDA Adverse Event Report Analysis
Detailed analysis of 20,453 reports from the FDA Adverse Event Reporting System (FAERS). Reports span 2004–2025.
Total Reports
20,453
Death-Related Reports
2,949
Hospitalization Reports
8,381
Top Indication
Product Used For Unknown Indication
Gender Distribution
Age Distribution
Most Reported Adverse Reactions (FAERS)
| # | Reaction | Reports |
|---|---|---|
| 1 | DRUG INEFFECTIVE | 2,670 |
| 2 | DRUG INTERACTION | 1,547 |
| 3 | SEIZURE | 1,382 |
| 4 | TOXICITY TO VARIOUS AGENTS | 1,353 |
| 5 | CONVULSION | 1,262 |
| 6 | OFF LABEL USE | 1,087 |
| 7 | STATUS EPILEPTICUS | 791 |
| 8 | STEVENS-JOHNSON SYNDROME | 770 |
| 9 | PYREXIA | 683 |
| 10 | FALL | 609 |
| 11 | DRUG REACTION WITH EOSINOPHILIA AND SYSTEMIC SYMPTOMS | 600 |
| 12 | DIZZINESS | 597 |
| 13 | CONDITION AGGRAVATED | 556 |
| 14 | SOMNOLENCE | 537 |
| 15 | EPILEPSY | 525 |
Reactions in Death Reports
Reactions in Hospitalization Reports
Source: FDA FAERS (Adverse Event Reporting System) FDA FAERS (Adverse Event Reporting System) Reports are voluntary and do not establish causation
Serious Warnings
This medicine can cause serious heart problems if given too quickly. The injection rate should not be faster than 50 mg per minute for adults, and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) for children. Your heart will be monitored closely during and after the injection.
Known Drug Interactions
Anticonvulsants carbamazepine , phenobarbital, primidone, phenytoin ↓ nirmatrelvir/ritonavir Co-administration contraindicated due to potential loss of virologic response and possible resistance [see Contraindications (4) ] .
Mechanism: Phenytoin makes the body get rid of the antiviral medicine too quickly. This prevents the drug from working properly and could allow the virus to become harder to treat.
What to do: Do not take these drugs together. Your doctor may need to change your seizure medicine or use a different antiviral.
Interaction Drug Interaction Rifabutin, phenytoin, efavirenz, cimetidine, esomeprazole* Avoid coadministration unless the benefit outweighs the risks ( 7.6 , 7.7 , 7.8 , 7.9 ) Other drugs metabolized by CYP3A4 Consider dosage adjustment and monitor for adverse effects and toxicity ( 7.1 , 7.10 , 7.11 ) Digoxin Monitor digoxin plasma concentrations ( 7.12 ) Fosamprenavir, metoclopramide* Monitor for breakthrough fungal infections ( 7.6 , 7.13 ) *The drug interactions with esomeprazole and metoclopramide do not apply to posaconazole tablets. 7.8 Phenytoin Phenytoin induces UDP-glucuronidase a...
Mechanism: Phenytoin causes the body to break down posaconazole much faster than usual, which can make the antifungal medicine less effective.
What to do: Avoid using these medicines at the same time unless your doctor decides it is necessary and monitors you closely.
CYP3A Inducers Do not use ranolazine with CYP3A inducers such as rifampin, rifabutin, rifapentine, phenobarbital, phenytoin, carbamazepine, and St.
Mechanism: Phenytoin makes your body clear ranolazine much faster than it should, which lowers the drug's effectiveness.
What to do: You should avoid taking these two medicines at the same time.
Carbamazepine (CYP450 Induction) Not Studied In Vivo or In Vitro , but Likely to Result in Significant Reduction Contraindicated Long Acting Barbiturates (e.g., phenobarbital, mephobarbital) (CYP450 Induction) Not Studied In Vivo or In Vitro , but Likely to Result in Significant Reduction Contraindicated Phenytoin (CYP450 Induction) Significantly Reduced Increase voriconazole maintenance dose from 4 mg/kg to 5 mg/kg IV every 12 hours or from 200 mg to 400 mg orally every 12 hours (100 mg to 200 mg orally every 12 hours in patients weighing less than 40 kg). Phenytoin (CYP2C9 Inhibition) Sig...
Mechanism: Phenytoin speeds up how fast the body gets rid of voriconazole, while voriconazole slows down the breakdown of phenytoin. This means the antifungal may not work well, and the seizure medicine could reach toxic levels.
What to do: Your doctor will need to increase your voriconazole dose and monitor your phenytoin blood levels very closely.
7.2 Combined P-gp Strong CYP3A4 Inducers Avoid concomitant use of apixaban tablets with combined P-gp and strong CYP3A4 Inducers (e.g., rifampin, carbamazepine, phenytoin, St.
Mechanism: Phenytoin causes your body to get rid of apixaban much faster than it should. This means the medicine may not work well enough to stop blood clots from forming.
What to do: Avoid using these two medicines together and ask your doctor for an alternative medication.
Common Questions
What should I tell my doctor before taking phenytoin?
Can I take phenytoin with other medicines?
What are the signs of an allergic reaction to phenytoin?
Will phenytoin cure my seizures?
Can I drink alcohol while taking phenytoin?
What happens if I stop taking phenytoin suddenly?
Does phenytoin affect birth control?
Can phenytoin cause gum overgrowth?
Will phenytoin affect my ability to drive or operate machinery?
How often will I need blood tests while taking phenytoin?
Does phenytoin interact with other medications?
What drug class is phenytoin?
Is phenytoin safe during pregnancy?
Related Medications in Anticonvulsant (Hydantoin)
Other drugs grouped near phenytoin — same-class peers and common alternatives.
brivaracetam
Briviact
Brivaracetam is a medicine used to treat partial-onset seizures.
Compare with phenytoin →
cannabidiol
Epidiolex
Epidiolex is a medicine that contains cannabidiol.
Compare with phenytoin →
carbamazepine
Tegretol
Carbamazepine is a medicine used to control seizures and treat nerve pain.
Compare with phenytoin →
cenobamate
Xcopri
Xcopri is a medicine used to treat partial-onset seizures in adults.
Compare with phenytoin →
clobazam
Onfi, Sympazan
Clobazam oral suspension is a medicine used with other medicines to treat seizures caused by Lennox-Gastaut syndrome.
Compare with phenytoin →
Medication Guides
Understanding Drug Interactions
How CYP450 enzymes, inhibitors, and inducers affect your medications
Generic vs Brand Name Drugs
FDA requirements, cost savings, and when the difference matters
Narrow Therapeutic Index Drugs
Why some drugs demand precise dosing and monitoring
Common Drug Interactions
Dangerous medication combinations and how to protect yourself
Related Health & Safety Data
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What the FDA Data Shows for phenytoin
The FDA label for phenytoin (sold under brand names such as Dilantin) classifies it as a prescription-only medication in the Anticonvulsant (Hydantoin) class. This medicine treats generalized tonic-clonic status epilepticus, a type of prolonged seizure. Labeling covers dosing, contraindications, and monitoring requirements derived from clinical trials.
Post-market surveillance from the FDA Adverse Event Reporting System (FAERS) captures real-world experience. For this drug, FAERS contains 12,150 voluntary reports. The database also lists 147 documented drug interactions derived from FDA labeling, with the top-flagged interaction rated major severity. NADAC pricing from CMS shows a generic unit cost of $1.61.
Report counts do not establish causation — a FAERS entry documents a temporal association, not proof that the drug produced the outcome. Widely prescribed medications naturally accumulate more reports than niche therapies, so raw totals must be interpreted alongside total exposure. Shortage status, recall history, and patent information further shape supply and switching decisions. This page summarizes public FDA data for educational purposes only and is not a substitute for professional medical advice — always consult a licensed healthcare provider before starting, stopping, or changing any medication.
Data Sources
Drug labeling: FDA Drug Labels (SPL/DailyMed). Adverse events: FDA Adverse Event Reporting System (FAERS). Pricing: CMS National Average Drug Acquisition Cost (NADAC).
FAERS reports are voluntary and do not establish causation. Drug interactions are derived from FDA labeling and clinical references. Always consult a healthcare professional before making medication decisions.
Last updated: April 9, 2024
Read our methodology — how this data is sourced, computed, and verified.
All federal data sources used on this page
- FDA Orange Book — approved drug products with therapeutic equivalence. accessdata.fda.gov/cder/ob
- FDA DailyMed — NIH-hosted drug labeling for FDA-approved meds. dailymed.nlm.nih.gov
- FDA Adverse Event Reporting System (FAERS) — post-marketing safety surveillance. fda.gov/drugs/faers
- NLM RxNorm — standardized clinical drug nomenclature. nlm.nih.gov/research/umls/rxnorm
- CMS Medicare Part B Drug Average Sales Price Files — federal drug pricing data. cms.gov/medicare/part-b-drugs/asp
- FDA Drug Shortages Database — current and resolved drug shortage tracking. accessdata.fda.gov/drugshortages