Sustained increases among homosexual men of unsafe sexual behaviour, sexually transmitted infections, and HIV have caused much concern.¹ HIV transmission among homosexual men continues despite the use of antiretroviral therapy that lowers infectiousness.² The idea that “treatment optimism” has led to “behavioural disinhibition” has attracted much attention although recent data suggest it is by no means the whole story.³

Guidelines for the administration of antiretroviral drugs as post-exposure prophylaxis following non-occupational or sexual exposure (termed nPEP in the United States and PEPSE in the United Kingdom) have been drawn up in both the United Kingdom⁴ and the United States.⁵ These guidelines offer advice on when to give prophylaxis and to whom. Both guidelines acknowledge that the evidence in support of the recommendations is not watertight. Much of the evidence is inferred from retrospective data on occupational exposure,⁶ the use of antiretroviral therapy to reduce mother to child transmission of HIV,⁷ and on experimental simian immunodeficiency virus (SIV) and HIV-2 infections in macaques.^8,9 The data suggest that PEPSE can reduce the risk of infection with HIV if administered promptly (less than 72 hours after exposure) and continued for 28 days. A non-randomised study among 202 homosexual men provided with …