In addition to this main objective, another important issue regards the known association between the use of nevirapine and the occurrence of certain untoward effects, including the development of rash, clinical hepatitis and a hypersensitivity syndrome which usually presents with rash, elevated alanine aminotransferase and/or aspartate aminotransferase, eosinophilia and a host of other constitutional symptoms. The possibility that a metabolite of nevirapine might be responsible for one or the other of these events has been evaluated but could not be confirmed. Data is equivocal on whether nevirapine levels themselves are associated with these events, probably because of "noise" in the retrospective analyses that was used. Animal models would suggest, though, that supratherapeutic nevirapine levels are associated with these events. It seems reasonable to assume that there is a sub-population of people at risk for all of these events (especially hypersensitivity) based on their genetic make-up in that it is known that many patients easily tolerate significantly elevated levels of nevirapine while others may get these side effects with relatively normal levels.