There is supportive evidence based on biologic possibility, animal & other studies that PEP can potentially decrease the risk of HIV acquisition if taken early enough. PEP is not given for exposures after 72 hours.
The treatment is not 100% effective and there have been cases of people who have still been infected with HIV despite receiving PEP.
PEP is not given to everyone because of the potential side-effects and also if the risk of the exposure is deemed to be extremely low (less than 1 in 10,000 chance) such that benefits of taking the medication outweigh the risks.
PEP is also not useful if you have had multiple high risk exposures over a prolonged period of time or continue to be at high risk for exposure. In such case, you may be more suitable for Pre-Exposure Prophylaxsis (PrEP), which is taken as a preventive daily medication. Please speak to your doctor to find out more.