Table 1.

FDA Phenytoin Dosage and Administration (2019)

Gene or gene variantDosing considerations
HLA-B*15:02 Consideration should be given to avoiding phenytoin as an alternative for carbamazepine in individuals positive for HLA-B*15:02.
The use of HLA-B*15:02 genotyping has important limitations and must never substitute for appropriate clinical vigilance and individual management.
CYP2C9 and CYP2C19If individual is phenytoin naïve, do not use phenytoin/fosphenytoin. Avoid carbamazepine and oxcarbazepine.a
If the individual has previously used phenytoin continuously for longer than 3 months without incidence of cutaneous adverse reactions, cautiously consider use of phenytoin in the future. The latency period for drug-induced SJS/TEN is short with continuous dosing and adherence to therapy (4–28 days), and cases usually occur within 3 months of dosing.b

This FDA table is adapted from (1).

From: Phenytoin Therapy and HLA-B*15:02 and CYP2C9 Genotype

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