Phenytoin
From SNPedia
Phenytoin is the generic name for an anticonvulsant drug used primarily in the management of complex partial seizures and generalized tonic-clonic seizures. Phenytoin is also used to prevent seizures following neurosurgery. Additionally, phenytoin is a class 1b antiarrhythmic that can be used to treat cardiac arrhythmias when conventional options have failed or after cardiac glycoside intoxication.The most common trade names for phenytoin are Phenytek and Dilantin.Wikipedia
Patients with CYP2C9*2 and especially CYP2C9*3 alleles, which are low-functioning, are likely to be more sensitive to phenytoin. The SNPs characterizing these two CYPs are rs1799853 and rs1057910.