Clinical trials can be an important sources of information for your review. Trials with null or negative results sometimes struggle to get published, resulting in a publication bias. By only using published trials in your meta-analysis, it is possible to overstate the positive effect of the intervention. Luckily there is an easy way to capture a huge percentage of the worldwide clinical trials in one search.
The Trials database in the Cochrane Library database (aka CENTRAL) imports on a monthly basis references from clinicaltrials.gov and the WHO International Clinical Trials Registry Platform (ICTRP) meta-register (contains records from 17 national/international registries). If you do find records of interest through this search, you may need to go to the individual registries to see if study results have been posted for those trials.