Legal and medical experts are questioning the decision of a judge in Colorado to allow James Holmes, the suspected gunman in the Aurora cinema shooting, to be tested with a "truth serum" should he plead not guilty by reason of insanity.

Judge William Sylvester ruled that in the event of Holmes pleading insanity his prosecutors would be permitted to interrogate him while he is under the influence of a medical drug designed to loosen him up and get him to talk. The idea would be that such a "narcoanalytic interview" would be used to confirm whether or not he had been legally insane when he embarked on his shooting spree on 20 July last year.

The precise identity of the drug that would be used has not been released, other than a statement that it would be "medically appropriate", but it would most likely be a short-acting barbiturate such as sodium amytal.

William Shepherd, chair of the criminal justice section of the American Bar Association, whose members include both prosecutors and defence lawyers, said that the proposed use of a "truth drug" to ascertain the veracity of a defendant's plea of insanity was highly unusual in the US. He predicted it would provoke intense legal argument relating to Holmes's right to remain silent under the fifth amendment of the US constitution.

"If a defendant loses his right to remain silent because the court has authorised the use of drugs that make him talk, that would raise all sorts of fifth amendment issues that both sides would have to address."

Shepherd also wondered whether some members of the trial jury would object to "the sound of the government forcing a truth serum down the throat of a defendant".

There are very few recorded instances of "truth drugs" being applied to criminal trials in the US. One such case was the 1959 prosecution in California of Raymond Cartier, who was charged with killing his wife after a drunken night out.

The defendant was given "truth serum" with the blessing of the court, but in his case the "narcoanalytic interview" was used to support the defence's contention that Cartier had been insane at the time he committed murder. With Holmes, the prosecution would administer it to try and prove the opposite conclusion.

Holmes has been charged with multiple counts of murder arising from the shooting at the Century 16 cinema in Aurora, Colorado in which 12 people were killed and 58 injured. On Tuesday, a plea of not guilty was entered for Holmes after his lawyer told the court that the defendant was not ready yet to enter his own plea.

The proposed use of a "truth drug" has also prompted a critical response from medical experts. Dr August Piper, a Seattle-based psychiatrist who has used sodium amytal to treat patients who were mute or in a catatonic state and who has written research papers on the subject, said that this was "not a royal road to the truth".

"First of all, people can still lie under the influence of amytal. More importantly, the person under the influence of the drug is susceptible to outside suggestion."

Piper also questioned whether such a method could be used to find out the truth of what happened retrospectively. Though short-acting barbiturates might be beneficial in illuminating Holmes's current state of mind, by opening him up to greater communication, it would be of doubtful use in determining his state of mind at the scene of the shooting eight months ago.

"To try and do this would be unlikely to yield useful information, and could pervert the course of justice by rendering the defendant susceptible to pressure," Piper said.

Steven Hoge, a clinical professor of psychiatry at Columbia University in New York, said that there was "no great evidence to support that narcoanalytic interviews lead to the truth by any means". He said that it was extremely rare to find the technique used in a criminal context.

Hoge has only once conducted a "narcoanalytic interview" himself – in the case of a man who had been so severely drunk he couldn't remember what had happened to him. The hope had been that he might recover memory, though it proved unsuccessful.

Most experimentation, Hoge added, was in similar areas of lost or repressed memories. But it was not clear in the case of the alleged Aurora shooter that memory was an issue that would be relevant in seeking to ascertain a diagnosis of sanity or insanity at the time of the rampage.

"It's hard to see what the value of this procedure would be, unless his memory of the shooting is in question," he said.