Today's world is fast paced and demanding. Advertisers send messages that being thin is fashionable. Parents prod their children to study hard and earn good grades. Supervisors urge employees to be more productive. The drive to be the best one can be—thinner, smarter, or faster—seems to be part of the American psyche.

For those who always feel pressured to achieve more, chemical stimulants may be appealing. This diverse group of drugs, containing dozens of stimulants, has one common denominator: All of them speed up the functions of the body, making people feel more alert.

However, stimulants vary widely in how much they accelerate body activities, the manner in which they are used, and their legal status.

The most commonly used legal stimulants are caffeine and nicotine, both of which are derived from plants. The best-known restricted...