Introduction Most studies conducted at general practices investigate complex interventions and increasingly use cluster-randomised controlled trail (c-RCT) designs to do so. Our primary objective is to evaluate how frequently complex interventions are shown to be more, equally or less effective than routine care in c-RCTs with a superior design. The secondary aim is to discover whether the quality of a c-RCT determines the likelihood of the complex intervention being effective.

Methods and analysis All c-RCTs of any design that have a patient-relevant primary outcome and with a duration of at least 1 year will be included. The search will be performed in three electronic databases (MEDLINE, EMBASE and the Cochrane Database of Systematic Reviews (CDSR)). The screening process, data collection, quality assessment and statistical data analyses (if suitably similar and of adequate quality) will be performed in accordance with requirements of the Cochrane Handbook for Systematic Reviews of Interventions. A feasibility project was carried out that was restricted to a search in MEDLINE and the CCTR for c-RCTs published in 1 of the 8 journals that are most relevant to general practice. The process from trial selection to data collection, assessment and results presentation was piloted. Of the 512 abstracts identified during the feasibility search, 21 studies examined complex interventions in a general practice setting. Extrapolating the preliminary search to include all relevant c-RCTs in three databases, about 5000 abstracts and 150 primary studies are expected to be identified in the main study. 14 studies included in the feasibility project (67%) did not show a positive effect on a primary patient-relevant end point.

Ethics and dissemination Ethical approval is not being sought for this review. Findings will be disseminated via peer-reviewed journals that frequently publish articles on the results of c-RCTs and through presentations at international conferences.

Trial registration number PROSPERO CRD201400923.