gateway drug
US /ˈɡeɪtweɪ drʌɡ/
UK /ˈɡeɪtweɪ drʌɡ/
1.
a drug whose use is thought to lead to the use of more dangerous drugs (such as heroin or cocaine)
:
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Marijuana is often considered a gateway drug by some.
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The theory of gateway drugs suggests a progression from less harmful to more harmful substances.