"Truth serum" is a colloquial name for any of a range of psychoactive drugs used in an effort to obtain information from subjects who are unable or unwilling to provide it otherwise. These include ethanol, scopolamine, 3-quinuclidinyl benzilate, midazolam, flunitrazepam, sodium thiopental, and amobarbital, among others.

Although a variety of such substances have been tested, serious issues have been raised about their use scientifically, ethically and legally. There is currently no drug proven to cause consistent or predictable enhancement of truth-telling. Subjects questioned under the influence of such substances have been found to be suggestible and their memories subject to reconstruction and fabrication. When such drugs have been used in the course of investigating civil and criminal cases, they have not been accepted by Western legal systems and legal experts as genuine investigative tools. In the United States, it has been suggested that their use is a potential violation of the Fifth Amendment of the U.S. Constitution (the right to remain silent). Concerns have also been raised through the European Court of Human Rights arguing that use of a truth serum could be considered a violation of a human right to be free from degrading treatment, or could be considered a form of torture. It has been noted to be a violation of the Inter-American Convention to Prevent and Punish Torture.

"Truth serum" was abused against psychotic patients as part of old, discredited practices of psychiatry and is no longer used. In a therapeutic context, the controlled administration of intravenous hypnotic medications is called "narcosynthesis" or "narcoanalysis". Such application was first documented by Dr. William Bleckwenn. Reliability and suggestibility of patients are concerns, and the practice of chemically inducing an involuntary mental state is now widely considered to be a form of torture.