How well does PrEP work?

PrEP is much more than 99% effective at stopping HIV infection, when taken correctly.

There are very few HIV infections when taken as prescribed. 

These cases are usually because HIV was resistant to the drugs in PrEP or HIV was missed in the pre-test.

• Very recent HIV infections (six weeks before the HIV test) are not always detected. This can be avoided by having another HIV test six weeks after either the most recent risk or after starting PrEP.
• There is only a small risk that HIV transmission can occur, but other cases are reported. For example, in 2023, a small number of cases were reported at a London clinic even though adherence was good. See link.

Last updated: 1 November 2023.