A gateway drug is the first one taken in a pattern of drug-taking that tends to progress from licit to illicit, from drugs with a mild effect to drugs with a powerful effect. The pattern is observable in the statistics of drugtaking, but the reason for it is much debated. There is a theory that if you start young, then you are more of a risktaker and more accustomed to drugs as you get older. There is also the possibility that the gateway drug produces biological changes in the brain which then make later drug use more likely. The step from licit drugs to illicit ones is an important one and there are various factors that could be influential there. Marijuana is regarded as the most common gateway drug, although alcohol at an early age is also significant.