Phenytoin

Phenytoin is used in the management of seizures.

Mechanism of action

• binds to sodium channels increasing their refractory period

Adverse effects

Phenytoin is associated with a large number of adverse effects. These may be divided into acute, chronic, idiosyncratic and teratogenic. Phenytoin is also an inducer of the P450 system.

Acute

• initially: dizziness, diplopia, nystagmus, slurred speech, ataxia
• later: confusion, seizures

Chronic

• common: gingival hyperplasia (secondary to increased expression of platelet derived growth factor, PDGF), hirsutism, coarsening of facial features, drowsiness
• megaloblastic anaemia (secondary to altered folate metabolism)
• peripheral neuropathy
• enhanced vitamin D metabolism causing osteomalacia
• lymphadenopathy
• dyskinesia

Idiosyncratic

• fever
• rashes, including severe reactions such as toxic epidermal necrolysis
• hepatitis