If no evidence is found at the top levels, move down the list looking for systematic reviews and then single studies of first cohort studies and then case-series or case-control studies.
Consider the publication date in the selection process.
If the systematic review you find was published a number of years ago
and found inconclusive evidence, then look for newer randomized trials
as your next step.
Small result sets can be filtered “by eye”. For larger result sets, use search filters.
In PubMed, you can access filters for Article Type, Publication Date, Age, Language,
and other filters on the left side of the search results page. If you do not see the
filters or options you want, you can click the Additional Filters button at the bottom
of the filters menu to add more items to the menu.