While there is compelling data to suggest that PEP after HIV exposure is effective, there have been cases where it has failed. Failure has often been attributed to the delay in receiving treatment (greater than 72 hours post-exposure), the level of exposure, and/or the duration of treatment (lack of adherence to the 28-day regimen). In addition, since the time and level of non-occupational exposures are self-reported, there is no absolute data on the administration timeframe to which PEP would be efficacious. The standard antibody window period begins after the last day of PEP treatment. People who received PEP are typically advised to get an antibody test at 6 months post-exposure as well as the standard 3 month test.
