A list of the most addictive drugs. These are the 20 most addictive substances on Earth. All of them should only be taken under a doctor's supervision, if at all. The level of addictiveness is based on two separate studies. Multiple other studies have also been considered. Some of these drugs have medicinal properties. But even if taken as medication, one should still be careful. Just because it's medicine, that doesn't mean it's not potentially dangerous for your health.