gateway drug

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A drug or other ingested substance, such as alcohol or marijuana, whose habitual use is thought to lead to the use of very addictive and dangerous drugs such as cocaine and heroin.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An addictive substance that is seen as relatively harmless by itself, but is believed to encourage the user to experiment with more harmful substances.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a habit-forming drug that is not addictive but its use may lead to the use of other addictive drugs