How well does PrEP work?

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

Tens of thousands of people take PrEP in the UK. Very few HIV infections occur when taken as prescribed.

The few cases are sometimes because:

  • HIV was resistant to the drugs in PrEP.
  • Very recent HIV infection (six weeks before the HIV test). In this case someone is HIV positive without realising it. This can be avoided by having another HIV test six weeks after either the last risk or after starting PrEP.

Last updated: 1 November 2022.