Assisted injection among injection drug users (IDUs) remains understudied. We recruited 1,056 IDUs using respondent-driven sampling in Tijuana, Mexico. Participants underwent HIV and syphilis testing and structured interviews. One-quarter (25%) sought injection assistance in the past 6 months. Seeking injection assistance was independently associated with being female (adjusted odds ratio [AOR]= 2.59; 95% confidence interval [CI]= 1.73-3.90), being born outside Baja California (AOR = 1.75; CI = 1.26-2.42), having recent abscesses (AOR = 2.59; CI = 1.93-3.47), using syringes previously used by others in the past 6 months (AOR = 1.99; CI = 1.45-2.71), and ever being arrested for carrying sterile syringes (AOR = 1.55; CI = 1.15-2.09).