gateway drug

English Thesaurus

1. a habit-forming drug that is not addictive but its use may lead to the use of other addictive drugs (noun.artifact)

hypernym	:	drug of abuse, street drug,
definition	:	a drug that is taken for nonmedicinal reasons (usually for mind-altering effects); drug abuse can lead to physical and mental damage and (with some substances) dependence and addiction (noun.artifact)