Main findings
In this nationwide cohort study, 46 of 83 included RCTs (55%) did not achieve their recruitment target within the pre-planned study period with a maximal extension period of six months. RCTs that had a no-treatment arm, low funding and low financial compensation per included patient were at risk to experience this type of recruitment failure, while a preceding pilot study lowered this risk. Upon extension of the pre-planned study period from six to twelve months, 41 RCTs (49%) still did not achieve the pre-planned recruitment target. Nineteen RCTs (23%) were stopped prematurely because of recruitment issues.