Individuals often start by taking just a few diet pills, but this behavior can easily spiral out of control into full-blown addiction. Caffeine, ephedrine and phenylpropranolamine are only a few of the addictive and dangerous substances that can be found in diet pills. It’s also important to note that herbal “alternatives” to diet pills can be just as deadly. These “natural” substances stimulate the central nervous system and can have potentially serious side effects.