Phenytoin hypersensitivity reaction:
- inherited enzyme def of epoxide hydroxylase. Phenytoin is aromatic.
- cutaneous reaction seen after 1 - 3 wks.
- Fever, diffuse erythematous purpuric, non-blanchable, cutaneous eruption.
- leukocytosis, hepatitis, nephritis.
Tx: stop phenytoin, start gluco-corticoids.
Valproic acid (non-aromatic) is safe. Cross reactivity with other drugs (aromatic) like carbamazepine, phenobarbital, felbamate.