The hypothesis of this study is that the development of red man syndrome (RMS) during receipt of intravenous vancomycin is associated with the presence of variant alleles for genes involved in the histamine pathway. The primary outcome that will be measured will be the history of RMS and the presence or absence of variant alleles for the genes responsible for histamine metabolism (i.e. histamine n-methyltransferase and diamine oxidase). As a secondary endpoint, the study will also attempt to determine the incidence of RMS in pediatric patients.