Hi there
Post Exposure Prophylaxis (PEP), as you have mentioned, is a course of anti-HIV medication which is taken for 1 month after a high risk sexual exposure to HIV. This aims to prevent the HIV virus from infecting you after you have been exposed to it.
PEP is normally given when the HIV exposure risk is assessed to be high ( e.g. there is a more than 1 in 10,000 chance that you might HIV from that single sexual encounter). During the consultation to assess if you need PEP, your doctor will ask you a detailed history on the nature of the sexual exposure, characteristics of the sexual partner ( e.g. whether his/her HIV status is known etc) so as to determine the level of risk.
You can refer to the table in the reference link from the US CDC to see the average risk per single sexual exposure. Generally, the highest risk comes from unprotected anal sex (receiving and giving) and the lowest risk from oral sex. Other factors like broken skin or cuts at the genital area of mouth may also influence the risk and decision for PEP as these increase the risk of exposure.
One of the reasons that PEP is usually only given for high risk exposures is that the medication does carry a small risk of side effects.
Another important point to note is that PEP must be started as soon as possible after the exposure. The effectiveness of starting PEP has only been shown to be within a 72 hour period, and if you miss this cutoff, there will be no benefit from taking the medication.
I hope you find this information helpful.
Dr Wei Sheng
Dermatologist and Sexual Health Specialist
