PrEP is more than 99% effective. There are very few infections when PrEP is taken 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.

However, like all medicines, some people might get some side effects. Mild nausea, diarrhoea, bloating and headache are reported in the first month by less than 1 in 10 people. These side effects then usually stop.

Occasionally, PrEP can cause more serious side effects that reduce kidney function and/or bone health. This is why kidney tests are needed when using oral PrEP.

Changes in bone structure are usually small and reverse after stopping TD/FTC. This is similar to a commonly used oral contraceptive but might be important if you are younger than 18 when bones are still developing. Bone strength was not fully reversed after a year in one study in adolescents aged 15 to 19.

PrEP does not increase the risk of bone fractures.