BACKGROUND: Chemsex is a colloquial term used by gay men in some parts of the UK to describe the use of psychoactive substances (typically mephedrone, GHB/GBL or crystal methamphetamine) during sex. Use of these drugs by gay men in London appears to have risen sharply from relatively low levels and, as yet, there is little data to inform appropriate harm reduction services. This study sought to understand the personal and social context of chemsex and the nature of harm reduction need. METHODS: In-depth interviews were conducted with 30 self-identifying gay men (age range 21-53) who lived in three South London boroughs, and who had used either crystal methamphetamine, mephedrone or GHB/GBL either immediately before or during sex with another man during the previous 12 months. Data were subjected to a thematic analysis. RESULTS: While around half of participants had utilised a range of drugs over many years, others had only recently been introduced to drugs, often by sexual partners who wished to enhance the sexual session. As relatively new drugs on the gay scene, understanding of appropriate dosing was lacking and a majority described overdoses, particularly in relation to GHB/GBL. Negotiation of sex, especially in group sex environments, was complicated by the effects of the drugs and a small number of men reported concerns relating to sexual consent. While a significant proportion of men had experienced a range of physical and mental health harms, few had accessed professional support for fear of judgement or concern about chemsex expertise. CONCLUSION: Findings from this study indicate a substantial degree of harm in the usage of relatively new psychoactive substances in highly sexual circumstances. Generic drug services, typically designed to address the needs of opiate users, may not be sufficiently resourced to address the specific and acute needs of gay men engaging in chemsex.