Narcoanalysis — a euphemism for the use of psychoactive drugs, or a truth serum — is regarded by many experts as unlikely to yield trustworthy results. Indeed, it may amount to torture.

Typically in India, where narcoanalysis is often used in serious crime investigations, a subject is attached to a drip containing the barbiturate sodium pentothal, which induces a trance-like state. While in this trance the subject is questioned by a forensic psychologist.

The evidence is inadmissible in Indian courts but the police insist that it is invaluable.

Truth serums are banned in most democracies. While drugs can make subjects more talkative, experts say that a subject with a firmly entrenched false story embedded in his mind can still lie. They also say that dosed subjects often mix fact with fantasy.

In Britain any evidence obtained in such a manner would be inadmissible on a number of counts, John Cooper, a criminal law lawyer, said. The use of a needle by police could constitute assault, a defendant under the influence of drugs is not considered fit to be questioned and the law prohibits the use of coercion during questioning.

A Stanford Law Review article in 2004 said: “In the United States, no law at either the state or national level makes the use of truth serum a crime per se.”

The Geneva Convention forbids its use against prisoners of war — but if terrorism suspects are not considered prisoners of war the conventions would not block it, the author suggested.

The CIA search for a truth serum, the Project MK-ULTRA, began in 1953 when the CIA tested the effects of several drugs. People were given substances without their knowledge or consent and one jumped to his death after taking the hallucinogen LSD.

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