Does the protection given by PrEP encourage people to use condoms less than before, or to have more sexual partners? Some people have been very concerned by this possibility.

The PROUD study was designed to help answer this question by mimicking real-life conditions in which people receiving PrEP knew they had the active, effective drug – not a placebo.

At the beginning of the study, participants reported an average of ten partners every three months. Most used condoms with some but not all of their partners. Many had recently used PEP and had sexually transmitted infections. The participants therefore had risky sexual behaviour before taking PrEP – it was why they felt they needed it.

For most of these men, sexual behaviour remained unchanged throughout the study, whether or not they were receiving PrEP. This suggests that most participants added PrEP to existing risk-reduction strategies, including condom use – they did not replace condoms with PrEP.

However, the sexual behaviour of a minority of men taking PrEP did change. The proportion of participants who reported receptive anal intercourse without a condom with large numbers of sexual partners increased somewhat. Nonetheless, this was not reflected in a higher rate of sexually transmitted infections in those receiving PrEP.

The iPrEx OLE study (which also mimicked real-life conditions) is reassuring on sexual behaviour. Uptake of PrEP was a little higher in people who already had riskier sex, but comparing periods when people were taking and not taking PrEP, there were no differences in terms of self-reported sexual behaviour or sexually transmitted infections.

Whereas condoms protect against HIV, gonorrhoea, chlamydia, syphilis, numerous other infections and unwanted pregnancy, PrEP only protects against HIV. (It may also have some impact on herpes simplex virus type 2 and hepatitis B, but this is uncertain). Rates of sexually transmitted infections have remained at high levels in gay men taking PrEP. For example, half the participants in PROUD had gonorrhoea, chlamydia or syphilis while in the study. There have also been a handful of cases of hepatitis C in HIV-negative gay men receiving PrEP.