It would be hard to imagine improving on the intelligence of computer engineer Bjoern Stenger, a doctoral candidate at Cambridge University. Yet for several hours, a pill seemed to make him even brainier. Participating in a research project, Stenger downed a green gelatin cap containing a drug called modafinil. Within an hour, his attention sharpened. So did his memory. He aced a series of mental-agility tests. If his brainpower would normally rate a 10, the drug raised it to 15, he said. "I was quite focused," said Stenger. "It was also kind of fun." The age of smart drugs is...