"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."
(Answers.com)
A synthetic chemical (Often produced by a high-tech lab) the purpose of which is to produce a reliable state of intoxication (As opposed to plant-based inebrients, whos effects may vary greatly from one individual to another)

to tell someone to calm down and relax.

Pills such as ecstasy