Sodium amytal (amobarbital), pentothal sodium (thiopental), and seconal (secobarbital) are a collective class of barbiturate that produce a range of depressant effects from mild sedation to deep anesthesia, and are used as truth serums.
Barbiturates act on higher brain functions by binding to GABA-A protein receptors which convey signals around the brain, thus allowing the influx of Cl- ions which hyperpolarise cells and inhibit the GABA receptors.
In small doses, barbiturates are sedatives that reduce anxiety and have been used to treat psychogenic disorders. But in larger doses, they act as anaesthetics that depress the central nervous system, resulting in a relaxed and cooperative subject.
Some believe that because lying is more complex than telling the truth, suppression of the higher cortical functions will yield results. However, the reliability of evidence is questionable since the subject is in a highly suggestive state and may confuse fantasy with reality.