Dependence Rating (Out of 3): 2.89

No surprise here: heroin’s addictiveness is the stuff of legend. As an opiate, it affects opioid receptors throughout the body and mimics endorphins, reducing pain and causing pleasure. Areas of the brain involved in reward processing and learning are stocked with tons of these opioid receptors, so when you inject heroin, you are basically training your brain to make you crave it. Pair that with nasty withdrawal symptoms and high fat solubility (which allows it to get into your brain quickly), and you have the most addictive drug in the world. An estimated 281,000 people received treatment for heroin addiction in the US in 2003, and according to the National Institute on Drug Addiction, a full 23 percent of people who have ever used heroin become addicts.