BACKGROUND: Drug injection is a major health-related problem worldwide. Injection cessation and relapse to injection could significantly alter the risk of HIV and hepatitis C virus (HCV) among people who inject drugs (PWID). This study aimed to estimate the rate of injection cessation and relapse to injection among PWID in Iran. METHODS: This cohort study was conducted from 2018 to 2021 in the cities of Kerman and Tehran. Using a respondent-driven sampling (RDS) approach, 118 PWID with a history of injection in the last six months and negative HIV and HCV tests were recruited. Follow-up visits occurred every three months over a period of one year. Participants were interviewed and tested for HIV and HCV using rapid tests. Injection cessation was defined as the no injection of any type of drugs in the last three months. Relapse to injection was defined as re-initiating drug injection among those who had ceased injection. Two separate Cox regression models were applied, and an adjusted hazard ratio (aHR) with a 95% confidence interval (CI) were measured to assess the factors associated with each outcome. RESULTS: The rate of injection cessation was 26.1 (95% CI: 21.3, 32.0) per 100 person-years, and the rate of relapse to injection was 32.7 (95% CI: 24.7, 43.2) per 100 person-years. At the baseline interview, 39.8% (n = 47) of participants reported injection cessation in the past three months before the interview. In the multivariable Cox regression analysis, the rate of relapse to injection was greater among women (aHR = 1.58; 95% CI: 1.01, 2.52), and those with higher monthly income (aHR = 1.63; 95% CI: 1.03, 2.59). However, there was no significant variable that predicted injection cessation. CONCLUSION: Injection cessation was common among PWID in Iran, however, one-third relapsed to injection shortly after cessation. Harm reduction programs should include comprehensive strategies to reduce the probability of relapse among PWID who achieve injection cessation.