Perhaps you should try some spelling variations:
"performance-enhancing drugs" » "performance-enhanced drugs", "performance-enhancing designs"
"performance-enhancing drug" » "performance-enhanced drug", "performance-enhancing design"
"performance-enhancing data" » "performance-enhancing a"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("performance-enhancing drugs") OR ((("performance-enhancing drug") OR ((("performance-enhancing data") OR ("performance-enhanced data")))))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"performance-enhancing drugs\") or (((\"performance-enhancing drug\") or (((\"performance-enhancing data\") OR (\"performance-enhanced data\"))))))".
• Removing quotes may allow a broader search: (( performance-enhancing drugs ) or ((( performance-enhancing drug ) or ((( performance-enhancing data ) OR ( performance-enhanced data )))))).
• Adding a wildcard symbol may retrieve word variants: (("performance-enhancing drugs") or ((("performance-enhancing drug") or ((("performance-enhancing data") OR ("performance-enhanced data"))))))*.