BACKGROUND AND AIMS: Injecting drug use is a chronic condition, with people who inject drugs (PWID) typically experiencing repeated cessations and relapses during their injection careers. We characterize patterns of ceasing and relapsing and the impact of opiate substitution treatment (OST) during the entire injecting careers of PWID in the Edinburgh Addiction Cohort (EAC). METHODS: During 2005-2007, 432 surviving participants of the EAC were interviewed about their injecting histories. Adjusted associations between covariates and hazards of cessation and relapse were estimated using random-effects models. RESULTS: OST was strongly associated with a higher hazard of cessation (HR = 1 .71, P < 0.001), but there was no significant evidence of association with hazard of relapse (HR = 0.81, P = 0.14). Women and older PWID were less likely to relapse (HR = 0.73, P = 0.02 and HR = 0.55, P < 0.001, respectively). Hazards of both cessation and relapse decreased monotonically with time since last relapse/cessation (both P < 0.001). An individual's hazard of cessation increased with his/her number of previous cessations (HR = 3.58 for 10+ previous cessations, P < 0.001), but there was no evidence that an individual's hazard of relapse changed with number of previous relapses (P = 0.37). There was heterogeneity in the individual hazards of both cessation and relapse. CONCLUSIONS: OST was associated with reduced time to cessation, and there was some suggestion of increased time to relapse too. The likelihood of prolonged cessation is greater for women, increases with age, and decreases with time since last relapse.