I thought they always said, "I am going to make you a deal you can't refuse."
What is "truth serum"?
It is a barbiturate called thiopental sodium, better known as Sodium Pentothal, a trademark of Abbott Laboratories. It is a yellow crystal that can be dissolved in water or alcohol and administered orally or intravenously.
What does it do?
Sodium Pentothal is used as a sedative and as an anesthetic during surgery. It depresses the central nervous system, slows the heart rate and lowers the blood pressure. Patients on whom the drug is used as an anesthetic usually are unconscious less than a minute after it enters their veins. The drug causes only a few minutes of sedation. Because of its effectiveness as a sedative, it is the first of three drugs used by U.S. prison systems during executions.
In milder doses, the drug becomes what some call a "truth serum." Those taking the drug become very communicative and share their thoughts without hesitation. Despite its nickname, Sodium Pentothal will not make a person tell the truth against his will. The recipient likely will lose his inhibition, and therefore he might be more likely to tell the truth.
Sources: Dorland's Illustrated Medical Dictionary and the American Medical Association