Preventive CYP2C9 and CYP2C19 genotyping may therefore hold the potential to improve the safety of phenytoin treatment, as incoordination, confusion, and motor dysfunction are highly dependent on phenytoin plasma concentrations.135 Moreover, even idiosyncratic adverse effects, such as Stevens-Johnson syndrome, appear to be related to CYP2C9 genotype, phenytoin dose, and plasma concentration.136,137 Feasibility and cost-effectiveness analyses of preemptive genotyping in phenytoin pharmacotherapy are needed to appropriately evaluate the clinical utility of such an intervention. This evidence concerns the gene CYP2C9 and Stevens-Johnson syndrome.