A "morning after" pill is due to be tested on people who may have been exposed to HIV the night before.
Doctors at New York's Beth Israel Medical Centre want to see if they can stop the virus from establishing itself in the hours after exposure.
Previous studies among health workers - exposed accidentally via needles - suggest immediate treatment can stop the virus replicating.
"The reality is that people sometimes do engage in high-risk behaviours or are accidentally exposed to HIV when a condom breaks," said researcher Dr Brian Saltzman. "We need to find a way to help reduce their risk."
Doctors plan to study various "morning after" drugs.
The treatment consists of a four-week course of drugs that are thought to reduce the likelihood of infection if they are taken within 72 hours of exposure to the virus.
Experience suggests less than 10 per cent of people develop signs of infection if they receive preventive AIDS treatments.
PA