Background: Characteristics and behaviors of early-onset injection drug users are under studied top-ics in Iran. This study aimed to identify and compare the demographic characteristics as well as thedrug using behaviors of early-onset and late-onset injection drug users in Kermanshah, West Iran.Methods: In this cross-sectional study using snowball and convenience sampling, we recruited 450people during the Fall of 2014 from two drop in centers in Kermanshah, Iran. We collected data throughface-to-faceinterviews.Early-onsetinjectionisdefinedaswhetherthepersonreportedtheirfirstinjec-tion at 22 years of age or younger. Subsequently, late-onset injection is defined as 23 years of age orolder. We compared the characteristics of the two groups through both univariate and multiple logis-tic analyses.Results: Overall, 54% (CI 95%: 44.3%, 62.2%) were early injectors. After controlling for lowsocioeconomic status, initiation of drug use at a young age, multiple drug use and methamphetamineuse were all significantly associated with a higher likelihood of early-onset injection. Additionally,early-onsetinjectionwasassociatedwithrecentsyringeborrowing(OR=2.6,p=0.001),recentsyringelending (OR=1.4,p=0.01), recent cooker sharing (OR=3.2,p=0.01) and injecting two or moretimes a day (OR=2.2,p=0.04).Conclusion: Early-onset injectors were more likely to report a lowersocioeconomic status, initiation of first drug use at a younger age, using methamphetamine along-side polydrug use, and engaging in higher risk taking behaviors like borrowing needles. With theseassociations, the study emphasizes the need for drug-prevention programs to focus on the transitionto injection drug use at younger ages