Background: Peer-to-peer injection (either providing or receiving an injection to/from a person who injects drugs [PWID]) is common (19%-50%) among PWID. Most studies of peer-to-peer injection have focused on receiving injection assistance, with fewer examining providing injection assistance and none considering characteristics of PWID who do both. We examined characteristics of PWID by peer-to-peer injection categories (receiving, providing, both, and neither) and determined if these behaviors were associated with receptive and distributive syringe sharing.Methods: Los Angeles and San Francisco PWID (N = 777) were recruited using targeted sampling methods and interviewed during 2011-2013. Multinomial logistic regression was used to determine characteristics associated with peer-to-peer injection categories and logistic regression was used to examine if peer-to-peer categories were independently associated with distributive and receptive syringe sharing.Results: Recent peer-to-peer injection was reported by 42% of PWID (18% provider; 14% recipient; 10% both). In multinomial regression analysis, PWID reporting any peer-to-peer injection were more likely to inject with others than those who did neither. Injection providers and those who did both were associated with more frequent injection, illegal income source, and methamphetamine injection while injection recipients were associated with fewer years of injection. Injection providers were younger, had more years of injecting, and were more likely to inject heroin than PWID who did neither. In multivariate analyses, we found that providers and *