truth serum

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

• noun Any of various hypnotic or anesthetic drugs, such as scopolamine or thiopental sodium, used to induce a subject under questioning to talk without inhibition.

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

• noun informal, medicine A psychoactive medication, especially one administered by injection, which supposedly renders a person cooperative and disposed to respond honestly to questions.

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

• noun an intravenous anesthetic drug that has a hypnotic effect; induces relaxation and weakens inhibitions; is believed to induce people to talk freely