Background: A quarter of randomised clinical trials (RCTs) are prematurely discontinued; mainly due to slow recruitment of patients. Up to 60% of such discontinued RCTs remain unpublished.

Objective: To qualitatively explore underlying mechanisms and root causes of trial discontinuation due to slow recruitment.

Methods: We conducted semi-structured interviews with 49 clinical trial stakeholders such as trialists (n=22), patient representatives (n=3), presidents of research ethics committees (n=4), heads of clinical trial units (n=6), representatives of national funding agencies (n=2), the pharmaceutical industry (n=10), and regulatory agencies (n=2) between August 2015 and November 2016. All interviews were conducted in English, transcribed, and qualitatively analysed using MAXQDA.

Results: Emerging themes regarding slow recruitment and trial discontinuation were: Limited availability of funding for clinical research in the academic setting; challenges for investigators in developing collaborations with multiple recruiting hospitals in a decentralised healthcare system; the way in which 'success in research' is measured for clinicians; and, scepticism from patients towards clinical trials. Our results revealed that over optimistic numbers of eligible and consenting patients due to a lack of empirical evidence from pilot studies are common and lead to prolonged patient recruitment and exhausted budgets, particularly in the academic setting, that eventually trigger trial discontinuation. Industry-sponsored RCTs typically overcome recruitment problems by exchanging insufficiently recruiting centres with new centres. Other frequently mentioned root causes for slow recruitment were inexperienced principal investigators, busy clinicians, and lack of a professional research infrastructure.

Conclusions: More careful planning and diligent monitoring of recruitment; established recruitment networks; access to professional infrastructure; and, adequate funding were identified as key factors to prevent recruitment failure. International collaborative efforts of the clinical research community are needed.