Unexpected effects of PEP in Amsterdam Unexpected effects of PEP in Amsterdam SIV (simian immunodefiency virus) is related to HIV, and in susceptible monkeys SIV causes an AIDS-like condition. Experiments with monkeys have found that giving them anti-SIV drugs shortly after they are exposed to SIV greatly reduces their risk of becoming infected with that virus. This use of medicines shortly after exposure to a germ to prevent infection is called post-exposure prophylaxis (PEP). Animal studies also suggest that PEP is most effective the sooner it is used after exposure, within a maximum of 72 hours post-exposure. PEP must be taken every day for four consecutive weeks to prevent HIV infection. In most high-income countries, including Canada, PEP is freely available after a work-related exposure such as a needle-stick injury among health care workers. However, the use of PEP is not generally subsidized for accidental exposure because of consensual sex in Canada. Still, PEP combined with intensive counselling about risk behaviour may be a useful way of helping to limit the spread of HIV among sexually active people who are at high risk for HIV infection, particularly men who have sex with men (MSM). Researchers in Amsterdam have been studying two groups of men, as follows: MSM who have received PEP because of possible exposure to HIV arising from consensual sex MSM who did not receive PEP but were being monitored to assess changes in their HIV infection status over time Over the past decade, the Dutch researchers found that