Recreational drug use refers to the intentional use of drugs for recreational purposes. A controlled substance is recreational use if it is used without medical advice or supervision to alter one’s mental state. There is usually recreational use of alcohol, nicotine (tobacco), caffeine (tea, coffee) and drugs included in the United Nations Single Convention on Narcotic Drugs and the Convention on Psychotropic Substances. Psychopharmacologist Ronald Keith Siegel, however, calls it the fourth drive. That is, he compares it to the other three human drives: hunger, thirst, and shelter. Therefore, according to him, the intense and lasting addiction to these drugs makes them as important to the user as food, water and shelter.