UNLABELLED: Background This paper establishes the prevalence of chemsex drug use among men who have sex with men (MSM), the extent to which these drugs are used in a sexual context, as well as their associated behaviours and circumstances of use. METHODS: Data from a cross-sectional, online survey of 2328 MSM recruited via gay sociosexual media in Scotland, Wales, Northern Ireland and the Republic of Ireland were analysed. RESULTS: While almost half (48.8%) of participants had ever taken illicit drugs, lifetime chemsex drug use was less common (18.0%) and far fewer reported chemsex drug use in the last year (8.2%) or last 4 weeks (3.0%). Just over one-quarter (27.1%) of men who used chemsex drugs in the last year reported no sexualised drug use, but almost three-quarters (72.9%) did. Only 6.1% of the whole sample reported sexualised chemsex drug use in the last year. The odds of reporting chemsex in the last year were significantly higher for men aged 36-45 years (AOR=1.96), single men (AOR=1.83), men who were HIV positive (AOR=4.01), men who report high-risk sex (AOR=4.46), being fisted (AOR=7.77) or had sex in exchange for goods other than money (AOR=4.7) in the last year and men who reported an HIV test in the last 3 months (AOR=1.53). DISCUSSION: Only a small proportion of MSM in Scotland, Wales, Northern Ireland and the Republic of Ireland reported chemsex, and, for the first time, it is demonstrated that not all chemsex drug use was sexualised. Nevertheless, MSM who engage in chemsex (MWEC) reported substantial sexual risk inequalities. These novel findings highlight several opportunities for intervention, particularly around the multiple vulnerabilities of MWEC, opportunities for early identification of those most vulnerable to chemsex-related harm and the potential to develop a specialised responsive patient pathway.