'Chemsex' refers to the combining of sex and illicit drugs, typically mephedrone, GHB/GBL, and crystal methamphetamine. While numerous studies have examined the role of illicit drugs in sexual risk taking, less attention has been paid to the broader social context and structures of their use among gay men. Given their established role in influencing health related behaviour, this study sought to examine the nature and operation of social norms relating to chemsex among gay men residing in South London.In-depth interviews were conducted with thirty 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. In addition, two focus groups (n=12) were conducted with gay men from the community to explore group-level perceptions of drug use and chemsex.Chemsex was perceived as ubiquitous amongst gay men by a majority of participants, who additionally described a variety of ways it is arranged (including mobile apps) and a variety of settings in which it occurs (including commercial and private settings). Chemsex was associated with unique sexual permissions and expectations, although participants also described having personal boundaries with respect to certain drug and sex practices, suggesting within-group stigmatisation.This study clearly documents exaggerated beliefs about the ubiquity of chemsex, shifts in the perceived normativity of certain settings and means to facilitate chemsex, and attitudes revealing stigma against certain types of chemsex and men who engage in it. There is a need for health promotion interventions to challenge social norms relating to drug use generally, and chemsex specifically, and for such interventions to make use of the online settings in which chemsex is often facilitated.