Phenytoin
A hydantoin sodium channel blocker for tonic-clonic and focal seizures and status epilepticus, with saturable metabolism and many interactions.
Also called diphenylhydantoin (older chemical name).
Drug card
- How it works
- Blocking neuronal sodium channels to stop rapid, repetitive firing
- Used for
- Tonic-clonic, focal and post-neurosurgery seizures
- Status epilepticus (IV)
- Trigeminal neuralgia, if carbamazepine fails
- Known for
- Nystagmus, unsteadiness and slurred speech
- Gum overgrowth (gingival hyperplasia)
- Excess body hair and coarse facial features
- Its name
- -toin means a hydantoin antiepileptic (phenytoin type)
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does phenytoin work?
Phenytoin, a hydantoin, blocks voltage-dependent sodium channels and so cuts sustained high-frequency firing. Its liver metabolism, mainly by CYP2C9, saturates at higher levels, so a small dose increase can cause a large jump in blood level. It is also a potent inducer of liver drug-metabolizing enzymes.
Like other sodium channel-blocking antiepileptics: These drugs block voltage-gated sodium channels in neurons in a use- and voltage-dependent way: the faster a neuron fires, the more channels they block. Ordinary signaling carries on, but the sustained high-frequency bursts that spread a seizure are cut short. Phenytoin and carbamazepine are also strong inducers of liver enzymes, so they speed the breakdown of many other drugs.
Uses
What is phenytoin used for?
- Tonic-clonic, focal and post-neurosurgery seizures
- Status epilepticus (IV)
- Trigeminal neuralgia, if carbamazepine fails
Side effects
Side effects of phenytoin
- Nystagmus, unsteadiness and slurred speechKnown for
- Gum overgrowth (gingival hyperplasia)Known for
- Excess body hair and coarse facial featuresKnown for
- Drowsiness and confusion
- Rash, including SJS and TENSerious
- Birth defects (fetal hydantoin syndrome)Serious
- Low blood pressure and arrhythmias with fast IV dosesSerious
- Bone thinning and osteomalacia
- Megaloblastic anemia
- Liver injurySerious
- Blood disorders, including agranulocytosisSerious
- Skin damage where an IV dose leaks (purple glove)Serious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Give IV doses slowly with cardiac monitoring: fast infusion can cause severe hypotension and arrhythmias.
- Narrow therapeutic range and saturable metabolism: monitor blood levels when doses or other drugs change.
- Avoid if possible in HLA-B*15:02 or CYP2C9*3 carriers, who are at higher risk of SJS/TEN.
- Not effective for absence seizures; it lowers levels of hormonal contraceptives, DOACs and many other drugs.
Good to know: Its kinetics are nonlinear: as the liver enzymes saturate, a dose rise of 10% or more can push levels into the toxic range.
Remember it
A way to remember it
Sodium channel blockers let single shots through but jam the machine gun.
Their block grows with each firing, so ordinary signals pass while the rapid, repetitive bursts of a seizure are stopped.
Written for biofideX
Its name
-toin means hydantoin antiepileptic (phenytoin type)
Phenytoin and fosphenytoin, its injectable prodrug, are hydantoins that block sodium channels. Nitrofurantoin, an antibacterial for urinary infections, shares the ending.
- Every drug name stem
Its class
Sodium channel-blocking antiepileptics
Other sodium channel-blocking antiepileptics with a card:
Easily mixed up with local anesthetics, valproate and succinimides.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Extended phenytoin sodium capsules: label (DailyMed)
- Phenytoin sodium injection: label (DailyMed)
- Phenytoin 100 mg film-coated tablets: SmPC (emc)
- Phenytoin (NCI Thesaurus)
- Fetal Hydantoin Syndrome (NCI Thesaurus)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Epilepsy and nerve pain
Phenytoin is one of 8 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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