According to the National Institute on Drug Abuse, no single factor can predict whether a person will become addicted to drugs. Some drugs, such as marijuana and heroin "have a similar structure to chemical messengers called neurotransmitters, which are naturally produced by the brain. This similarity allows the drugs to “fool” the brain’s receptors and activate nerve cells to send abnormal messages", while others, such as cocaine, reward the brain with dopamine, which "sets in motion a reinforcing pattern that “teaches” people to repeat the rewarding behavior of abusing drugs" (http://www.drugabuse.gov/publications/drugfacts/understanding-drug-abuse-addiction). However, other factors such as individual biology, social environment, and age can all affect addiction.
An article, titled "The 10 Hardest Drugs to Kick", details the findings from a Dutch study, where scientists determined a dependency rating that measured addictive potency of drugs. The list includes (from higher dependency to less):
Heroin, Crack Cocaine, Nicotine, Methadone, Crystal Meth, Alcohol, Cocaine, Amphetamines, Benzodiazepines (anti-anxiety), and GHB.
-Kristen (Future Librarian)