In one study, adolescents who smoked cigarettes were about twice as likely as nonsmokers to later use marijuana. for this reason, cigarettes have been referred to as

For this reason, cigarettes have been referred to as a gateway substance.
A gateway substance is a medical term that refers to something, usually tobacco, or drugs, which leads to the use of something heavier and more dangerous. So, these adolescents moved from smoking cigarettes to smoking pot. 

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