It is probably fair to say that all of us experiment with recreational drugs at some time or other in our lives. For the most part, however, these drugs are socially acceptable and do not become problematic: Drinking alcohol in social situations to relax, taking caffeine in the form of tea or coffee to increase alertness, or smoking the occasional cigarette (inhaling nicotine) to calm ourselves down, are all examples of “normal” drug use. These substances all exert psychoactive effects, that is, they alter our mental functioning via an effect on brain activity. Indeed, this is often the very reason for which we take them. They may, however, be taken in excessive amounts, and then can lead to a range of medical, social, and psychological problems.