Illicit drug use before or during sex - known as sexualized drug use (colloquially 'chemsex' or 'party and play') - has evolved as novel psychoactive substances have entered the market in many parts of the world. Here, we review key conceptual issues in associations between illicit drug use and sexual risk-behaviour in MSM.Although many studies have confirmed that MSM use drugs with greater prevalence than the general population, evidence is of variable quality and a sampling frame is difficult to establish. Moreover, psychosocial hypotheses linking drug use and sexual risk, including cognitive escape and sensation seeking, are unsatisfactory and generally ignore strategic use of drugs for sexual aims. Person-level associations between drug use history and both sexual risk behaviour and HIV infection tend to be consistent around the world, but evidence comparing encounters within subjects is generally unclear and out of date.There is a need for interventions for harm reduction targeted at MSM that account specifically for the social and cultural contexts of sexualized drug use. Expanded attention to surveillance of emerging drug use trends can help clinicians in sexual health and infectious diseases best anticipate the needs of their service users.