It was the most sensational murder case of the year and after a series of embarrassing bungles, Indian police were under pressure to get results.

So they turned to a practice long since banned in most democracies, but on the rise in India: they injected their prime suspects with a “truth serum”.

India has been transfixed by the murder of Aarushi Talwar, 14, who was found with her throat slit in May at her home near Delhi. Police initially blamed the Talwars' domestic help, but were forced to rethink when his body was found on the terrace of the family house the next day.

Then they detained Rajesh Talwar, the dead girl's dentist father, and drugged him with sodium pentothal — the “truth serum”. The Central Bureau of Investigation, India's equivalent of the FBI, took over and declared him innocent last week.

The CBI now says that the culprit was Krishna, an assistant in Dr Talwar's clinic, who was subjected to six hours of “narcoanalysis” at the Forensic Science Laboratory (FSL) in Bangalore. A suspected accomplice is now receiving the same treatment.

The Aarushi case has exposed the incompetence of Indian police and aroused fears among middle-class Indians that they can no longer trust their increasingly disgruntled domestic staff. But it has also stoked a national debate about the police's use of narcoanalysis, polygraphs and brain mapping — often in the absence of proper forensic investigation.

It comes as the Supreme Court is poised to deliver its long-awaited verdict on whether narcoanalysis violates the Indian Constitution.

The practice is illegal in Britain, the United States and most other Western democracies, although security officials have suggested that it should be used on suspected terrorists — and some allege that it already has been. India adopted the technique in 2000 when S.Malini, a doctor who is now assistant director of the FSL in Bangalore, used it to coax evidence from a witness in another murder case.

Prisoners are usually taken to a specialist forensic laboratory, where doctors give them sodium pentothal, a commonly used anaesthetic, through a drip to induce a trance-like state. A forensic psychologist then questions the prisoner during the trance, which typically lasts from 15 to 45 minutes.

Although any evidence gleaned is inadmissible in court, police say the technique is an invaluable and harmless way of establishing facts. “It helps the investigating officer to reach the depths of the crime so that justice and law can prevail,” Rajan Bhagat, a police spokesman, told The Times.

However, human rights and medical ethics advocates accuse the police of using narcoanalysis as a substitute for proper criminal investigation. They say that it violates the Constitution, which prohibits anyone accused of an offence from being “compelled to be witness against himself”.

Some say that it is unethical for doctors to take part in narcoanalysis, since the drug tends to be administered against the prisoner's will and can cause respiratory or cardio-vascular complications.

Doctors often have to slap the prisoners to keep them awake, according to rights groups. “This is nothing but torture,” said Amar Jesani, a co-founder of the Forum for Medical Ethics Society.

Times Recommends

• No apology for Germany’s own Nazi victims
• Intricate web of fear and hope tying down deal
• Victims remembered a year after Mumbai attacks