Hi Medeina, thats a tricky question because scientists have a hard time measuring this! Some ways they try to estimate this is by taking surveys and look at where people have gone to hospitals looking for treatment. One way to study this more “biologically” is to see how easily a rat will learn to self-administer the drug. This means, the rat learns to push a lever in a box to have the drug injected directly into him. In these sort of tests, drugs such as heroine come out on top. Also from the human studies and surveys its seems this is the hardest drug to kick. Drugs are chemicals, and heroine in particular acts on the brain very quickly and is very good at tricking the brain cells into changing so the brain will want to experience heroine again in the future.