Wiktionary
gateway drug
n. An addictive substance that is seen as relatively harmless by itself, but is believed to encourage the user to experiment with more harmful substances.
WordNet
gateway drug
n. a habit-forming drug that is not addictive but its use may lead to the use of other addictive drugs; "one college athlete recently called beer a gateway drug for young people"
Usage examples of "gateway drug".
Talking about how healthy my self-image is, is like the gateway drug to pure psychobabble.