THIS drug is peddled on every street corner in America, and is found in every country in the world. It is psychoactive, a stimulant and addictive. Users say that it increases alertness and focus, and reduces fatigue. But the high does not last and addicts must keep consuming it in increasing quantities. Put this way, sipping coffee sounds more like an abomination than the world's most accepted form of drug abuse. But centuries of familiarity have put people at their ease. In the coming years science is likely to create many novel drugs that boost memory, concentration and planning. These...