This is a linkpost for https://worldchess.com/2017/01/25/special-report-new-study-finds-performance-enhancing-drugs-for-chess/
Modafinil improved performances by 15 percent, methylphenidate by 13 percent, and caffeine by around 9 percent."
"Dr. Lieb said that the researchers went into the study with the expectation that the stimulants would not show much benefit. “We primarily thought that it is not possible to enhance high cognitive tasks and were astonished to find such results,” he said."
I'm not surprised. But I also don't see much utility from this study; most people already believe that coffee helps them focus.