Patients receiving opioid substitution treatment (OST) experience social vulnerability, stigma, and barriers to healthcare access. Although organized cancer screening programmes are free and widely available in France, little is known about participation rates and barriers among patients treated with OST in primary care settings.To assess participation in organized cancer screening programmes (breast, cervical, and colorectal cancers) among patients receiving OST in Eastern France, and to identify barriers and factors associated with non-participation.We conducted a multicentre cross-sectional quantitative study between July 2024 and June 2025 in the Bas-Rhin and Haut-Rhin regions. Adult patients receiving methadone or buprenorphine were recruited in addiction treatment centres, general practices, hospitals, and a prison setting. Data were collected using two structured questionnaires assessing socio-demographic characteristics, screening participation, and perceived barriers. Descriptive analyses were performed, followed by univariate and multivariate logistic regression to identify factors associated with non-participation.Seventy-seven patients were included (45.5% women; mean age 49.8 years). Overall, 67.5% were not up to date with at least one recommended cancer screening. Among eligible participants, participation rates were 62.9% for cervical cancer, 40.0% for breast cancer, and 21.2% for colorectal cancer. The main barriers reported were low prioritisation of screening (35.7%), fear of cancer diagnosis (14.3%), misunderstanding of invitation letters (11.9%), perceived procedural burden (10.7%), and negative healthcare experiences (8.3%). In multivariate analysis, lower educational level and age between 45 and 55 years were independently associated with non-participation.Despite universal access to organized screening, patients receiving OST showed markedly lower participation than the general population, particularly for colorectal and breast cancer. Barriers were predominantly psychosocial and organisational rather than financial, highlighting the central role of health literacy and care coordination. The higher uptake of cervical screening may reflect more frequent contact with healthcare professionals. Selection and recall biases may have led to overestimation of participation, reinforcing the robustness of the observed inequalities.This study highlights persistent cancer screening inequalities among patients receiving OST in primary care. Integrating proactive counselling, adapted information, and outreach or on-site screening within addiction and general practice settings could improve participation and promote equity in preventive care.