How well does PrEP work?

PrEP is more than 99% effective. Very few people become HIV positive when taking PrEP as prescribed.

These rare cases are usually because HIV was missed in the pre-test.

• Recent HIV infections (six weeks before the HIV test) are not always detected. This is why it is good to have another HIV test six weeks after either the most recent risk or after starting PrEP.
• A small number of cases are because the HIV that someone was exposed to was already resistant to the drugs in PrEP.

Last updated: 1 February 2025.