There are definitely drugs out there, prescription and non, that can enhance your memory, mental stamina, and temporarily increase your focus.
You better believe that if you are a good chess player, this stuff can make you better temporarily. In case you haven't heard about these drugs, just do a search on New York Times or Google and you'll get a whole bunch of articles written about how college students use them to succeed on final exams and/or term papers, and how some students use it on standardized exams for higher education (SAT, GMAT, LSAT, MCAT, GRE etc)
fischer sipped on apple juice throughout his games. easily digested, so it doesn't take blood from your brain to your stomach, plus the sugar is essential. i like blueberry juice--very good for your brain--or a drink called "focus".