designer drug

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A drug with properties and effects similar to a known hallucinogen or narcotic but having a slightly altered chemical structure, especially such a drug created in order to evade restrictions against illegal substances.

Wiktionary

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

WordNet 3.0

1. n. 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