Gender, sexual orientation identity, and initiation of amphetamine injecting among people who inject drugs: Examination of an expanding drug era in Montreal, Canada, 2011–19

•Prevalence of amphetamine injecting increased four-fold among PWID in Montreal.•Gay or bisexual orientation doubled risk of initiating amphetamine injection in men.•Opioid injection and opioid agonist treatment associated with initiation in men.•Cocaine injection and sexual relations with PWID associated with initiation in women.