The concept of a gateway drug is not new. Most people have heard of the idea, but may not know which substances qualify as gateway drugs. It is important for everyone to be wary of gateway drugs. Part of this means understanding the risk factors of addiction and the slow progression toward physical dependence. This brief guide addresses three of the most common gateway drugs that anyone should be wary of: Alcohol Marijuana Prescription medication. These are the three most common gateway drugs. They are substances considered by many professionals and organizations to be dangerous.