Strategies to narrow down the number of results in a keyword search:
Add another concept using the 'AND' Boolean Operator. This will exponentially reduce the number of results you get back while making those results far more pertinent.
Reduce the number of synonyms you are using.
Try a field search. For example, run a title field search. Here the database will only look for your keywords in the title of an article. If the keywords you are looking for appear in the title ...then the chances of that article being pertinent to you are high.
Use the 'NOT' boolean operator to select words that you do not wish to appear anywhere in the article.
Check your Limiters, for example
set a date range or reduce your date range
limit your results to a specific language
limit your results by article type, e.g. randomized controlled trials or systematic reviews
Strategies to broaden up the number of results in a keyword search:
'OR' is always more. add more synonyms to your search string.
Reduce the number of concepts in your search string by eliminating 'AND' Boolean operators.
Phrases are very specific. reduce or replace phrases with single keywords if possible.