Drug-related harms, including harms from sexualised drug use (SDU), are disproportionately experienced by sexual and gender minority people, relative to their majority counterparts. Chemsex, a type of SDU practiced mainly by MSM, is associated with methamphetamine use and increased HIV seropositivity or risk of acquisition. Therefore, participants are at increased risk of immunocompromise. Existing evidence suggests that drug use increases following natural disasters. The impact of coronavirus disease 2019 (COVID-19) on chemsex is unknown. A PRISMA-adherent systematic review was conducted to synthesise reports of changes in the prevalence, frequency, or characteristics of drug use (and factors associated with these changes) following the onset of the COVID-19 pandemic. This report presents findings related to SDU/chemsex among MSM. A comprehensive search across nine databases, supplemented with backward-forward citation searching and contact with key opinion leaders, was conducted. Two reviewers carried out title-abstract screening, full-text screening, and data extraction. Following a final, single database search, nine studies were included in the narrative synthesis. More than half the sample were studies investigating HIV pre-exposure prophylaxis use. Twenty percent of participants in most studies reported chemsex participation. In four, participants reported a net increase or maintenance of chemsex participation during the pandemic and five reported a net decrease. Increased chemsex participation was associated with loneliness, cravings, and working during the pandemic. Decreased chemsex practice was associated with COVID-19-related fear. This synthesis suggests that chemsex practice continued, and for some MSM increased, throughout COVID-19 pandemic 'lockdowns'. This may have increased COVID-19 transmission and severity among potentially vulnerable MSM.