Clinical trials are registered in national registries to ensure transparency, reduce research waste and to enable follow up with researchers.
When searching for studies, the Cochrane Handbook for Systematic Reviews of Interventions Version 6 advises that multiple trial registries should be searched, including the Cochrane Central Register of Controlled Trials (accessed via the Cochrane Library). See Handbook sections 4.3.2 and section 1.2.1 in the Technical Supplement - Searching and selecting studies.
See How to cite a record on a clinical trials register for advice on citing trials data.