BackgroundPragmatic trials, because they study widely used treatments in settings of routine practice, require intensive participation from clinicians who determine whether patients can be enrolled. Clinicians are often conflicted between their therapeutic obligation to patients and their willingness to enroll them in trials in which treatments are randomly determined and thus potentially suboptimal. Refusal to enroll eligible patients can hinder trial completion and damage generalizability. In order to help evaluate and mitigate clinician refusal, this qualitative study examined how clinicians reason about whether to randomize eligible patients.MethodsWe performed interviews with 29 anesthesiologists who participated in REGAIN, a multicenter pragmatic randomized trial comparing spinal and general anesthesia in hip fracture. Interviews included a chart-stimulated section in which physicians described their reasoning pertaining to specific eligible patients as well as a general semi-structured section about their views on clinical research. Guided by a constructivist grounded theory approach, we analyzed data via coding, synthesized thematic patterns using focused coding, and developed an explanation using abduction.ResultsAnesthesiologists perceived their main clinical function as preventing peri- and intraoperative complications. In some cases, they used prototype-based reasoning to determine whether patients with contraindications should be randomized; in others, they used probabilistic reasoning. These modes of reasoning involved different types of uncertainty. In contrast, anesthesiologists expressed confidence about anesthetic options when they accepted patients for randomization. Anesthesiologists saw themselves as having a fiduciary responsibility to patients and thus did not hesitate to communicate their inclinations, even when this complicated trial recruitment. Nevertheless, they voiced strong support for clinical research, stating that their involvement was mainly hindered by production pressure and workflow disruptions.ConclusionsOur findings suggest that prominent ways of assessing clinician decisions about trial randomization are based on questionable assumptions about clinical reasoning. Close examination of routine clinical practice, attuned to the features of clinical reasoning we reveal here, will help both in evaluating clinicians’ enrollment determinations in specific trials and in anticipating and responding to them.Trial registrationRegional Versus General Anesthesia for Promoting Independence After Hip Fracture (REGAIN). ClinicalTrials.gov NCT02507505. Prospectively registered on July 24, 2015.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13063-023-07445-3.