The pharmacokinetics of fosphenytoin sodium injection could be affected by many factors, such as the narrow therapeutic index and large inter-patient pharmacokinetic variability of phenytoin; the variation of CYP2C9, which is one of the main enzymes involved in the metabolism of phenytoin; and the variation of HLA-B*1502, which is known to be related to an increased risk of life-threatening Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) in response to phenytoin treatment. The gene discussed is CYP2C9; the disease is toxic epidermal necrolysis.