Sited from FDA:
FDA is investigating new preliminary data suggesting an increased risk of serious skin reactions from the anti-epileptic drugs phenytoin and fosphenytoin if they are taken by Asian patients who are positive for the human leukocyte antigen allele HLA-B*1502. These reactions include Stevens Johnson Syndrome (SJS) and toxic epidermal necrolysis (TEN). Phenytoin is marketed as Dilantin, Phenytek and generics. Fosphenytoin sodium is marketed as Cerebyx and generics.
It is estimated that in parts of China, Taiwan, Thailand, Malaysia, Indonesia and the Philippines, 15 percent or more of the population may carry the HLA-B*-1502 allele. The frequency in South Asia, including India, is somewhat lower, and in Japan and Korea it is under one percent.