BACKGROUND Infective endocarditis (IE) in people who inject drugs (PWID) is an emergent public health problem. OBJECTIVES The purpose of this study was to investigate IE in PWID and compare it with IE in non-PWID patients. METHODS Two prospective cohort studies (ICE-PCS and ICE-Plus databases, encompassing 8,112 IE episodes from 2000 to 2006 and 2008 to 2012, with 64 and 34 sites and 28 and 18 countries, respectively). Outcomes were compared between PWID and non-PWID patients with IE. Logistic regression analyses were performed to investigate risk factors for 6-month mortality and relapses amongst PWID. RESULTS A total of 7,616 patients (591 PWID and 7,025 non-PWID) were included. PWID patients were significantly younger (median 37.0 years [interquartile range: 29.5 to 44.2 years] vs. 63.3 years [interquartile range: 49.3 to 74.0 years]; p < 0.001), male (72.5% vs. 67.4%; p = 0.007), and presented lower rates of comorbidities except for human immunodeficiency virus, liver disease, and higher rates of prior IE. Amongst IE cases in PWID, 313 (53%) episodes involved left-side valves and 204 (34.5%) were purely left-sided IE. PWID presented a larger proportion of native IE (90.2% vs. 64.4%; p < 0.001), whereas prosthetic-IE and cardiovascular implantable electronic device-IE were more frequent in non-PWID (9.3% vs. 27.0% and 0.5% vs. 8.6%; both p < 0.001). Staphylococcus aureus caused 65.9% and 26.8% of cases in PWID and non-PWID, respectively (p < 0.001). PWID presented higher rates of systemic emboli (51.1% vs. 22.5%; p < 0.001) and persistent bacteremia (14.7% vs. 9.3%; p < 0.001). Cardiac surgery was less frequently performed (39.5% vs. 47.8%; p < 0.001), and in-hospital and 6-month mortality were lower in PWID (10.8% vs. 18.2% and 14.4% vs. 22.2%; both p < 0.001), whereas relapses were more frequent in PWID (9.5% vs. 2.8%; p < 0.001). Prior IE, left-sided IE, polymicrobial etiology, intracardiac complications, and stroke were risk factors for 6-month mortality, whereas cardiac surgery was associated with lower mortality in the PWID population. CONCLUSIONS A notable proportion of cases in PWID involve left-sided valves, prosthetic valves, or are caused by microorganisms other than S. aureus. (C) 2021 by the American College of Cardiology Foundation.