The public health impact of widespread availability of nonoccupational postexposure prophylaxis against HIV. 
The aim of the study was to describe the use of nonoccupational postexposure prophylaxis (NPEP) in Australia, and to estimate the number of HIV infections that its use prevented.  We conducted a population-based observational cohort study of people who presented to antiretroviral prescribers in Eastern Australia, and reported a high-risk nonoccupational exposure to HIV, in 1998-2004.  Prescribers collected data at baseline, 4 weeks and 6 months.  Data collected included details of HIV exposure, drug regimens and HIV serostatus.  The great majority of the 1601 participants were male (95%) and presented after male homosexual exposure (87%).  Only 32% of exposures were to HIV-positive sources.  Two antiretroviral drugs were prescribed after 48% of events, and three or more drugs after 52% of events.  The median time to receipt of NPEP was 23 h. Side effects were reported by 66% of participants.  No case of NPEP failure in an adherent individual was identified.  It was estimated that 0.9-9.2 HIV infections had been prevented.  This compared with a total of 1138 newly acquired HIV infections notified in the geographical area covered by the study.  In Australia, NPEP has been widely prescribed and is mainly targeted at high-risk exposures.  Although there were no identified failures of NPEP, it is likely that only a small proportion of new HIV infections in the study area were prevented.  NPEP may be a valuable preventive intervention for an individual, but it can only play a minor role in HIV prevention at the population level unless targeting can be further improved.