A gateway drug is considered to be a type of habit forming drug which itself may not be addictive in nature but their use may lead to the person getting addicted to other drugs.
The increase in probability of using other illegal drugs when a person is using a gateway drug is because of the alteration in the brain which leads to an inclination towards drug usage and a carefree behavior.
Besides alcohol, marijuana and tobacco are also considered to be gateway drugs.
It has been shown that the use of alcohol acts a gateway for many drugs such as cocaine.
The gateway drug theory has stated if any of the gateway drugs or a mixture of them is introduced into a life of a teenager then it leads to an increased chances of experimentation with other drugs in the future.
Since, alcohol is linked to an increased probability of using illegal drugs in the future, it is considered to be a gateway drug.