gateway drugnoun [ C ]
uk/ˈɡeɪt.weɪ ˌdrʌɡ//ˈɡeɪt.weɪ ˌdrʌɡ/a drug whose use may make it more likely that someone uses other, more dangerous, drugs:
Early intervention could prevent adolescents from "graduating" from so-called gateway drugs such as alcohol and marijuana to harder drugs such as meth.
something that makes you more likely to try other things that are similar but more unusual or difficult:
Stephen King was my gateway drug to serious literature.