Extensive research about the topic has been conducted. After the publications of several studies in 1975, scientific discussions took a big lead in the theory. Different theories of the causes were developed. The two main theories were similar attitudes & alterations in the brain. The concept of similar attitudes is that several environmental & personal factors can increase the interest in different drugs. In the event of first use of a drug it would permit a look at the given social & economic environment. Alterations in the brain have been summed up in the sequence of an earlier drug use. Indications directly linked alteration consisting of lasting anatomical behaviors in the reward system of the brain.