Toxic epidermal necrolysis is characterized by significant – more than 30% - tissue damage from epidermal edema and bullae formation. There is a strong genetic component, so patients with first degree relatives (like mothers) who had this reaction should not receive phenytoin. Stevens-Johnson syndrome is less severe, so this is acceptable. Anaphylaxis in the maternal aunt is less concerning; the relation is farther removed and the genetic predisposition has been implicated in blistering reactions, rather than anaphylaxis. Phenytoin, if administered too quickly, can cause hypotension and subsequent circulatory collapse. The sibling’s event appears to be related to infusion rate rather than hypersensitivity.