Some people argue that marijuana acts as a gateway drug.
gateway drug
idiomA typically legal or less harmful substance, such as marijuana, whose use is statistically associated with an increased likelihood of progressing to a more dangerous or illicit drug.
The health expert warned that using nicotine products can sometimes serve as a gateway drug for teenagers.
While the debate remains contentious, many policymakers fear that legalizing certain substances will create a gateway drug that leads users toward more dangerous addictions.