Nicotine
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One of the most accessible legal drugs, nicotine is also the most addictive. "Most people who smoke just a few times will get hooked," says Kolodny. "Only a very few people are able to smoke occasionally without becoming addicted."

Whether you smoke cigarettes, cigars or chew tobacco, once you're addicted to nicotine, quitting can be exceedingly difficult. "It's harder to quit nicotine than alcohol," Maxwell says. "There continues to be such a strong urge to use."

However, unlike alcoholics who should go cold turkey, people trying to nix nicotine can turn to gum or patches to help ease them off the drug. "These keep level of nicotine in body while you get used to not smoking," says Maxwell. Still, most ex-smokers relapse. And even though many former smokers swear by the new smokeless e-cigarettes, these are not approved by the Food and Drug Administration, and it's unclear whether they too have long-term health ramifications.