BACKGROUND: Opioid substitution therapy (OST) is required for patients with opioid use disorder, but specialized addiction services do not reach all patients. General practitioners (GP) can fill this gap and provide a low-threshold, locally accessible offering for OST. This study aims to explore barriers that prevent GPs from offering OST. METHODS: In 2022, an online questionnaire with 31 items was sent to GPs in the federal state of Baden-Württemberg by e-mail. The items included demographic data, training and experience in addiction medicine, implementation of OST in practice and possible barriers and facilitators to OST in the primary care setting. The results were analysed using a quantitative and semi-qualitative approach. RESULTS: In total, 309 GPs participated (response rate = approx. 6.2 %). 26.2 % of participants had special training in addiction medicine (n = 81). 71.9 % of GPs with special training performed OST. 15.6 % of GPs without special training knew about reimbursement for and organization of OST (30/192). The barriers most frequently mentioned included: perceived lack of competencies, difficult patient clientele, and insufficient interdisciplinary communication. 22.4 % of GPs without training considered offering OST, provided that barriers are reduced. CONCLUSIONS: GPs have little knowledge of OST but one out of four considers offering OST. Thus, low-threshold addiction medicine courses in medical training and in continuing medical education are required. Barriers, such as bureaucratic obstacles and knowledge deficits, need to be addressed to decrease the growing gap in the care for patients with opioid use disorder.