The science behind why it's so difficult to quit smoking is crystal clear: Nicotine is addictive—reportedly as addictive as cocaine or heroin. Yet any adult can stroll into a drug store and buy a pack of cigarettes, no questions asked.

"From a scientific standpoint, nicotine is just as hard, or harder, to quit than heroin but people don't recognize that," said Dr. Neil Benowitz, a nicotine researcher at the University of California, San Francisco.