Remember, for keywords to work they have to reflect the concepts you want to investigate and they have to appear prominently in the documents you're looking for.
To effectively keyword search you have to find out how the people you are interested in talk about your research.
Acetaminophen is the active ingredient in the pain-reliever brand Tylenol. In a professional medical database, a search on Tylenol retrieved 123 results, while a search for acetaminophen retrieved 13,470 results. Acetaminophen is the best keyword to use in this scholarly database.
The average person would use the term Tylenol more often than acetaminophen. When looking for sources in news or other popular media, this is the better choice.