From today's NYT:
Caffeine, it turns out, actually works. And it is legal, one of the few performance enhancers that is not banned by the World Anti-Doping Agency. So even as sports stars from baseball players to cyclists to sprinters are pilloried for using performance enhancing drugs, one of the best studied performance enhancers is fine for them or anyone else to use. And it is right there in a cup of coffee or a can of soda.