Prescribing advice for phenytoin has been updated to reflect new evidence about the risk of a rare but serious side effect in patients of Thai or Han Chinese origin.
Studies have shown that the presence of the HLA-B*1502 allele in these individuals is associated with increased susceptibility to the potentially life-threatening skin reaction Stevens-Johnson syndrome.
HLA-B*1502 is just one of many genetic variants within the human leukocyte antigen (HLA) system, which collectively allow the immune system to recognise a broad range of foreign antigens. HLA-B*1502 is found in 8–9% of the Thai and Han Chinese populations.
Data are currently insufficient to recommend screening for the allele in patients from these ethnic groups before initiating phenytoin, as is currently advised with carbamazepine; however, phenytoin should be avoided if HLA-B*1502 is known to be present.