Key Points Question What is the prevalence of past 3-month cannabis use, and what are the reasons for use among patients in a large health system? Findings In this cross-sectional study of 175 734 patients, 17.0% reported cannabis use, among whom 34.7% had results indicative of moderate to high risk for cannabis use disorder. While most patients (76.1%) reported using cannabis to manage a health symptom, very few patients identified as medical cannabis users. Meaning Given the high rates of cannabis use, especially for symptom management, and the high levels of disordered use, it is essential that health care systems implement routine screening of primary care patients. Abstract...