Evidence supporting the use of PrEP to protect against HIV, how international scientific studies show it offers almost 100% protection from HIV when taken properly, and the use of real-life studies.

We base our PrEP advice on evidence from clinical studies done in the UK and across the world. These have involved scientists, researchers and thousands of volunteers.

The studies looked at how PrEP protected different groups of people, taking different doses of PrEP.

Evidence from these studies show that PrEP is a significant breakthrough in the fight against HIV.

PrEP offers you almost 100% protection from HIV, if you take it as instructed.

Also ground-breaking are the results of the PARTNER 2 study, released in July 2018. This confirms that Undetectable = Untransmittable for gay men as well as heterosexuals.

People on effective HIV treatment with an undetectable viral load can’t pass it on.

PrEP studies timeline

New way to reduce risk: iPrEx Study 2010

The iPrEX study showed that PrEP could be a new way to reduce the risk of gay men acquiring HIV.

It tested daily dosing, of 1 PrEP pill every day, for people at high risk of infection.

Just under 2,500 men who have sex with men (MSM) took part, as well as 29 transgender women. The volunteers were in Brazil, Ecuador, Peru, South Africa, Thailand and the USA.

It was a randomised controlled trial. This put participants, at random, in either:

  • a group taking PrEP, or
  • a group taking a placebo – something which has no medical effect.

Participants didn’t know which group they were in and neither did the people running the study. This approach means peoples’ opinions, or bias, did not affect the results.

iPrEX led to PrEP’s use in USA.

iPrEx also involved:

  • intensive counselling to help participants reduce their HIV risk
  • free condoms
  • interviews about behaviour
  • sexual health monitoring

Protection for trans women: iPrex-OLE 2014

iPrex-OLE continued the iPrEx study. It gave information on the safety of PrEP and looked at the behaviour of people taking PrEP for longer.

Its findings suggested that PrEP protected transgender women, if taken as instructed.

iPrEX-OLE was an open-label study. This means the participants and the researchers knew they were taking PrEP and not a placebo.

PrEP works in real-life setting: PROUD Study 2015

PROUD showed that daily dosing with PrEP is highly effective in a real-life setting.

Participants knew they were taking PrEP and did not increase risk-taking.

PROUD was an open-label trial carried out at 13 sexual health clinics in England.

At first PROUD was randomised, with participants chosen at random to receive PrEP either immediately, or after one year. But early in the trial, everyone taking part was given access to PrEP.

PROUD evidence showed how PrEP would be a cost-effective way to fight the HIV epidemic in the UK.

On Demand dosing: IPERGAY Study 2015

IPERGAY supported a cheaper, more flexible way to use PrEP.

The French/Canadian study looked at Event Based Dosing (EBD) / On Demand Dosing. This is individualised to the times people had sex. There were no infections in people who took PrEP as instructed.

Together with the iPrex and iPrex-OLE results, this study suggested that 4 doses of PrEP a week gave more than 96% protection for men. The study mainly looked at MSM, both receptive and insertive.

IPERGAY-OLE confirmed that PrEP was very effective at preventing HIV infection in MSM at higher risk and having frequent sex. There was a 97% relative reduction of HIV incidence in the group of men who took part.

#cantpassiton: PARTNER study 2016 and PARTNER 2 2018

The PARTNER and PARTNER 2 studies found that the chance of any HIV positive person with an undetectable viral load transmitting the virus to a sexual partner was scientifically equivalent to zero.

These findings confirm and validate Undetectable equals Untransmittable


The PARTNER study found that no infections happened during 58,000 condomless penetrative sex acts between 888 couples. This was hugely important.

The PARTNER study:

  • included gay and straight couples
  • measured risk in people not using condoms
  • estimated absolute risks.

The study was carried out on 75 clinical sites in 14 European countries.

From the PARTNER study, we know people living with HIV cannot pass on the virus during sex without condoms if they:

  • have been on treatment for 6 months
  • take their medication as instructed
  • have an undetectable viral load.

PARTNER 2 (2014 – 2018) recruited only gay couples. With 972 gay male couples and 77,000 condomless sex acts, the outcomes were the same.

The results show, in the words of the researchers ‘a precise rate of within-couple transmission of zero’ for gay men as well as for heterosexuals.

Principal researcher Alison Rodger told aidsmap.com: ‘We looked so hard for transmissions. And we didn’t find any.’

According to the study result, you would need to be having condomless sex for 435 years before there would be a chance of one transmission. It’s safe to say that the risk is zero!

Zero transmissions means zero risk.

This is called Treatment as Prevention or TasP.