Drug hypersensitivity reactions do not occur in everyone who uses a given drug, and a variety of factors may increase the risk. In general, women are more susceptible, as are older individuals^[1]. People with more intact immune systems seem more prone to certain types of immune-mediated hypersensitivity. However, the rate of hypersensitivity is higher in people with HIV^[2]. Individuals who have experienced drug-induced hypersensitivity reactions in the past are more likely to do so again, as are people with a history of allergic conditions such as asthma.

For abacavir (Ziagen) hypersensitivity, risk factors seen in various studies include male sex, African ethnicity, more advanced HIV disease and history of allergies^[3]. In one 50-person study, 18% of individuals with primary HIV infection developed a hypersensitivity reaction^[4].

Nevirapine (Viramune) hypersensitivity is more common in women and people with higher CD4 cell counts. Other risk factors include low body weight, co-infection with hepatitis B or C and certain genetic patterns^[5][6].

Several types of hypersensitivity appear linked to genetic predisposition^[7]. Most patients with abacavir hypersensitivity carry the HLA-B*5701 haplotype^[8][9]. This genetic pattern may be present in as many as 10% of the population, and is more common amongst Caucasians than people of African descent. For more information, see Abacavir hypersensitivity reaction.