designer drug

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

• noun A drug synthesized to have properties similar to a known hallucinogen or narcotic but having a slightly altered chemical structure, usually in order to evade restrictions against illegal substances.
• noun A drug that is synthesized, especially by genetic engineering, in order to treat a specific medical condition or a specific patient population.

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

• noun A drug specifically developed to replace an illegal recreational drug (so as to circumvent existing drug laws), usually by modifying its molecular structure.

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

• noun a psychoactive drug deliberately synthesized to avoid anti-drug laws; mimics the effects of a banned drug; law was revised in 1986 to ban designer drugs