An important category of grey literature, particularly for clinical research, is clinical trials. While many trials are reported in the published literature and can be located via Cochrane Central Register of Controlled Trials (CENTRAL), others are unpublished or ongoing. Searching registers for ongoing trials and trials with negative results is critical.
Finding out about unpublished studies and including them in a systematic review, when eligible and appropriate, is important for minimising bias. Ongoing trials should be identified and tracked for possible inclusion in reviews on completion.
Registers will not include records for all trials conducted as this is not always a legal obligation, however in many cases registration is required under organisational, funding, legal or publisher requirements.
World Health Organization's International Clinical Trials Registry Platform
Includes records from many key registers from around the world, including among others:
You may find there are advantages to searching individual registers as these will have more sophisticated search functionality and will be more current (some months may have elapsed since the last load of records to ICTRP) .
Australian New Zealand Clinical Trials Registry (ANZCTR)
European Union Clinical Trials Register
'If study registers (such as ClinicalTrials.gov)...were searched, specify the name of each source and any date restrictions that were applied.' - PRISMA 2020 Explanation and Elaboration, p. 5