Phenytoin has a narrow therapeutic index and large variability in blood level between people, partly because CYP2C9 clears it and CYP2C9 activity varies. Separately, HLA-B*15:02 is associated with an increased risk of Stevens–Johnson syndrome and toxic epidermal necrolysis on phenytoin — the same allele that carries that risk with carbamazepine. CPIC issued combined guidance in 2021 covering both genes: one governs how much drug there is, the other governs whether it is safe to start at all.
CPIC Guideline for CYP2C9 and HLA-B Genotypes and Phenytoin Dosing (Clin Pharmacol Ther 2021, PMID 32779747).
This page is educational and contains no dosing information. Where a clinical guideline covers one of these gene-drug pairs it is written for prescribers and works through validated algorithms alongside clinical monitoring. Nothing here is a reason to start, stop, or change a medication — that conversation belongs with the clinician or pharmacist managing your treatment.