Generating evidence for the efficacy of an intervention is not sufficient to guarantee its implementation in real-world settings. The randomized AMBORA trial (Medication Safety with Oral Antitumor Therapy) demonstrated that an intensified clinical pharmacological/pharmaceutical care program has substantial benefits for patients, treatment teams, and the healthcare system. Thus, we are now investigating its implementation into routine care within the AMBORA Competence and Consultation Center (AMBORA Center). We perform a multicenter type III hybrid trial following the RE-AIM framework to assess the clinical effectiveness of this care program under real-world conditions, while evaluating the implementation outcomes. Semi-structured stakeholder interviews based on the Consolidated Framework for Implementation Research (CFIR) have been conducted to identify barriers and facilitators. So far, 332 patients treated with oral antitumor drugs have been referred to the AMBORA Center by 66 physicians from 13 independent clinical units. In 20 stakeholder interviews (e.g., with clinic directors), 30% (6/20) of the interviewees anticipated possible barriers which may partly hinder sustainable implementation (e.g., unavailable consultation rooms). Furthermore, important facilitators (e.g., operational processes) were identified. This methodological description adds knowledge on how to structure a hybrid effectiveness–implementation trial and proposes multilevel implementation strategies to improve the medication safety of oral antitumor therapy.