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Objective To explore the approach to participant coenrolment in publicly funded randomized controlled trials (RCTs) and to consider its impact on study recruitment. Study Design and Setting A cross-sectional study of the UK National Institute for Health Research Journals Library was undertaken. RCTs funded between 2010 and 2019 were eligible. The reporting of coenrolment criteria was assessed through inspection of publicly available study protocols. Where present, the approach to coenrolment was examined, including circumstances in which it was permitted/prohibited and the mechanism for decision-making. For completed RCTs, the impact on recruitment was explored by comparing rates of early recruitment (completion before the expected end date) and extensions (completion after the expected end date) between studies, which did and did not permit coenrolment. Results Of 219 eligible protocols, coenrolment was addressed in 94 (42.9%). Twenty-three (24.5%) of these did not allow recruitment to multiple studies, while 71 (75.5%) permitted it according to a series of caveats, including considerations of study outcomes, intervention type, and patient burden. The final decision for coenrolment rested with the local recruitment team in 57 (60.6%) and with the central organizing team in 37 (39.4%). Early completion of recruitment occurred in 8 of 64 (12.5%) RCTs where coenrolment was permitted and 5 of 20 (25.0%) where it was not (P = 0.285). An extension to recruitment time was required in 31 of 64 (48.4%) RCTs where coenrolment was permitted and 9 of 11 (45.0%) where it was not (P = 0.788). Conclusions The reporting of coenrolment in protocols of publicly funded RCTs is infrequent, and where present, the approach to decision-making is widely variable. In this study, policies of coenrolment were not associated with gains in trial recruitment.