A systematic review is an analysis of all primary literature that exists on a specific topic. Primary literature includes only original research articles. Narrative reviews, systematic reviews, or meta-analyses are based on original research articles, and hence are considered as secondary sources. Therefore, you should not use these in the data extraction process for your systematic review. However, you can definitely use the original research articles cited by these sources.
We are conducting a systematic review (qualitative) on ketamine use in depression and its short- and long- term safety. Many reviews, and some meta-analyses have already been conducted on the topic - they have been located via our database searches. Should we include these in the data extraction process or just the original articles/research cited?