Two medicines, side by side
Fosphenytoin vs phenytoin
Fosphenytoin and phenytoin are both labeled for seizures and epilepsy. Drugs@FDA records approvals of fosphenytoin since 1996 and of phenytoin since 1953. How each works, in its label's words: Fosphenytoin, “Fosphenytoin is a prodrug of phenytoin and accordingly, its anticonvulsant effects are attributable to phenytoin.”; Phenytoin, “The precise mechanism by which phenytoin exerts its therapeutic effect has not been established but is thought to involve the voltage-dependent blockade of membrane sodium channels resulting in a …”
Check it at the source: the fosphenytoin FDA label on DailyMed (effective May 18, 2025); the phenytoin FDA label on DailyMed (effective October 15, 2024).
Highlights of the fosphenytoin record
Fosphenytoin (Cerebyx) · label of May 18, 2025
CARDIOVASCULAR RISK ASSOCIATED WITH RAPID INFUSION RATES The rate of intravenous CEREBYX administration should not exceed 150 mg phenytoin sodium equivalents (PE) per minute in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients because of the risk of severe hypotension and cardiac arrhythmias. Careful cardiac monitoring is needed during and after administering intravenous CEREBYX.
Used for
From the label: indicationsCEREBYX is indicated for the treatment of generalized tonic-clonic status epilepticus and prevention and treatment of seizures occurring during neurosurgery. CEREBYX can also be substituted, short-term, for oral phenytoin. CEREBYX should be used only when oral phenytoin administration is not possible [see Dosage and Administration (2.4) and Warnings and Precautions (5.2) ]. […]
On the market
- Earliest approval on file
- August 5, 1996
- Generic labelers
- 7 labelers
- Pharmacy cost
- Not in the survey
Supply and safety
- Shortage
- No current entry
- Recalls
- 6
Highlights of the phenytoin record
Phenytoin (Dilantin) · label of October 15, 2024
Used for
From the label: indicationsPHENYTEK ® capsules (extended phenytoin sodium capsules) are indicated for the treatment of tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. […]
On the market
- Earliest approval on file
- January 6, 1953
- Generic labelers
- 29 labelers
- Pharmacy cost
- $0.145 per capsule
Supply and safety
- Shortage
- No current entry
- Recalls
- 11
Source: FDA drug labels via openFDA (Fosphenytoin label of May 18, 2025, Phenytoin label of October 15, 2024); NDC Directory, Drugs@FDA, NADAC, FDA shortages and recalls (October 8, 2026)
How do fosphenytoin and phenytoin differ on the record?
How each works, in its label's words: Fosphenytoin, “Fosphenytoin is a prodrug of phenytoin and accordingly, its anticonvulsant effects are attributable to phenytoin.”; Phenytoin, “The precise mechanism by which phenytoin exerts its therapeutic effect has not been established but is thought to involve the voltage-dependent blockade of membrane sodium channels resulting in a …”
Only the fosphenytoin label carries a boxed warning.
Both are in FDA's class anti-epileptic agents.
Half-life, in each label's words: Fosphenytoin, “Elimination The conversion half-life of fosphenytoin to phenytoin is approximately 15 minutes.”; Phenytoin, “Elimination The plasma half-life in man after oral administration of phenytoin averages 22 hours, with a range of 7 to …”
Generics: fosphenytoin since 2007; phenytoin since 1975 (Drugs@FDA).
| Fosphenytoin | Phenytoin | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Seizures and epilepsy |
| Pharmacologic class | Anti-epileptic Agent | Anti-epileptic Agent |
| How it works (label) | Fosphenytoin is a prodrug of phenytoin and accordingly, its anticonvulsant effects are attributable to phenytoin. | The precise mechanism by which phenytoin exerts its therapeutic effect has not been established but is thought to involve the voltage-dependent blockade of membrane sodium channels resulting in a reduction in sustained high-frequency … |
| Half-life (label) | Elimination The conversion half-life of fosphenytoin to phenytoin is approximately 15 minutes. | Elimination The plasma half-life in man after oral administration of phenytoin averages 22 hours, with a range of 7 to 42 hours. |
| Earliest approval on file (Drugs@FDA) | August 5, 1996 | January 6, 1953 |
| First generic approved | August 6, 2007 | July 16, 1975 |
| Generic labelers (NDC) | 7 labelers | 29 labelers |
| Pharmacy cost (NADAC, each one's most listed form; strengths differ) | n/a | $0.145 per capsule (Phenytoin sod ext 100 mg capsule) |
| 12-month price change | n/a | -5% |
| Boxed warning | Yes | No |
| Shortage now | No current entry | No current entry |
| Recalls recorded | 6 | 11 |
| FAERS reports (a report is not proof of cause; counts follow how widely a medicine is used) | 2,553 | 40,278 |
| DEA schedule | none | none |
Source: Drugs@FDA, FDA NDC Directory, CMS NADAC of October 7, 2026, FDA shortages and recalls, FAERS via openFDA (FDA files October 8, 2026)
Do their labels mention each other?
Fosphenytoin label, boxed warningCARDIOVASCULAR RISK ASSOCIATED WITH RAPID INFUSION RATES The rate of intravenous CEREBYX administration should not exceed 150 mg phenytoin sodium equivalents (PE) per minute in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients because of the risk of severe hypotension and cardiac arrhythmias.
Fosphenytoin label, contraindicationsCONTRAINDICATIONS CEREBYX is contraindicated in patients with: A history of hypersensitivity to CEREBYX or its inactive ingredients, or to phenytoin or other hydantoins [see Warnings and Precautions (5.6) ].
Source: FDA drug labels via openFDA
What next?
- Alternatives to fosphenytoin
- every medicine labeled for the same conditions