Chemsex briefing from Harm Reduction International. Using substances for sex and socialising is not a new phenomenon. Drug use was documented among gay men and other men who have sex with men long before the term “chemsex” was coined. Chemsex, which has come to the attention of public health professionals in the past decade, involves both sexual and drug-related high risk behaviours such as multiple sexual partners, the use of multiple drugs together, and injecting drug use.