http://en.wikipedia.org/wiki/Paul_Erdős

"After 1971 he also took amphetamines, despite the concern of his friends, one of whom (Ron Graham) bet him $500 that he could not stop taking the drug for a month. Erdős won the bet, but complained during his abstinence that mathematics had been set back by a month: "Before, when I looked at a piece of blank paper my mind was filled with ideas. Now all I see is a blank piece of paper." After he won the bet, he promptly resumed his amphetamine habit."

What do you recommend? I drink coffee, I don't do anything else. What are others' experiences, both legal and illegal? I'd like to be able to get long stretches of difficult work done with greater concentration and faster thinking. Legality and supply in different countries is a factor. Please forgive me anonymously registering a handle to ask this question, and please feel free to yourself if your usage patterns are something you feel is private. Thanks.