Sodium thiopental is a rapid-onset short-acting barbiturate general anaesthetic. Thiopental is an ultra-short acting barbiturate and is most commonly used in the induction phase of general anesthesia. Following intravenous injection the drug rapidly reaches the brain and causes unconsciousness within 30–45 seconds. At one minute, the drug attains a peak concentration of about 60% of the total dose in the brain. Thereafter, the drug distributes to the rest of the body and in about 5–10 minutes the concentration is low enough in the brain such that consciousness returns. Instead, anesthesia is usually maintained with an inhaled anesthetic agent. This class of drugs has a relatively rapid elimination, so that stopping the inhaled anesthetic will allow rapid return of consciousness.

In addition to anesthesia induction, thiopental can be used to induce medical comas. Even though the drug is described as an ultra-short acting barbiturate, the drug's half-life is much longer and giving a larger dose ensures adequate concentrations in the brain to maintain anesthesia. Patients with brain swelling, causing elevation of the intracranial pressure, either secondary to trauma or following surgery may benefit from this drug.

Thiopental is sometimes used intravenously for the purposes of euthanasia. Along with pancuronium bromide and potassium chloride, thiopental is used in 37 states of the U.S. to execute prisoners by lethal injection. A megadose is given which places the subject into a rapidly induced coma. Executions using the three drug combination are usually effective in approximately 10 minutes, but have been known to take several times this length. The use of thiopental alone is hypothesized to cause death in approximately 45 minutes. Thiopental is still used in some places as a truth serum. The barbiturate drugs as a class decrease higher cortical brain functioning. Psychiatrists hypothesize that because lying is more complex than the truth, suppression of the higher cortical functions may lead to the uncovering of the "truth." However the reliability of confessions made under thiopental is dubious; the drug tends to make subjects chatty and cooperative with interrogators, but a practiced liar or someone who has a false story firmly established would still be quite able to lie while under the influence of the drug.

Formal Chemical Name (IUPAC)
sodium 5-ethyl-5-(2-methylpentyl)-4,6-dioxo-1,4,5,6-tetrahydropyrimidine-2-thiolate

References

http://en.wikipedia.org/wiki/Thiopental