Crystal Meth, GHB/GBL and Mephedrone form what some health workers call an 'un-holy trinity' of drugs that together can heighten arousal and strip away inhibitions. They've become increasingly popular on London's gay scene, and the effects can see some users taking part in weekend-long sex parties, involving multiple partners. For Radio 4's The Report, Mobeen Azhar speaks to men entrenched in this lifestyle and explores the impact the so-called 'chemsex' scene is having on public health services. It's a scene where unsafe sex is common and has been cited as a contributing factor in the rising number of HIV infections in London, posing new challenges to those trying to promote the safe sex message. Such parties are fuelled by technology and smartphone dating apps, which have triggered a social shift where men have moved out of bars and clubs and into private homes - out of reach to sexual health and drug advice services. A potential solution to help protect those involved in the scene is Pre Exposure Prophylaxis (PrEP) - the practice of issuing men with HIV medication before they become HIV positive, which studies have shown as an effective means to reduce HIV infection. PrEp has been championed by the World Health Organisation, saying it could prevent 1 million new HIV infections around the world. Its advocates in Britain suggest it should be made available on-demand as soon as possible - but how affordable is it? And will fears that it will only encourage more unsafe sex prove true? Presenter: Mobeen Azhar Producer: Richard Fenton-Smith.