DENVER—The defendant in a Colorado movie theatre massacre could be given “truth serum” under a court order issued Monday to help determine whether he is insane if he pleads not guilty by reason of insanity.

Suspect James Holmes could be required to submit to a “narcoanalytic interview” as part of an evaluation to determine if he was legally insane at the time of the July 20 shootings, Arpahoe County District Judge William Sylvester said.

A narcoanalylitic interview is a decades-old process in which patients are given drugs to lower their inhibition. Academic studies have shown that the technique has involved the use of sodium amytal and pentothal, sometimes called truth serum.

Holmes, 25, is scheduled to enter a plea on Tuesday to multiple counts of murder and attempted murder. He is charged with killing 12 people and injuring 70 at a midnight showing of “The Dark Knight Rises” in the Denver suburb of Aurora.

If Holmes pleads not guilty by reason of insanity, he would be examined by doctors at the state mental hospital.

The prospect of a narcoanalytic interview that may ensue under such a plea alarmed defence attorneys, who filed documents opposing the technique.

In an advisory that Holmes would have to sign if he enters an insanity plea, Sylvester didn’t specify what type of drugs would be used but said the examination could include “medically appropriate” ones.

Sylvester said Holmes also could be given a polygraph examination as part of the evaluation.

After reading a draft of the advisory, Holmes’ lawyers objected to the possibility of both a narcoanalytic interview and a polygraph, saying they would violate their client’s rights.

In the final version of the advisory, Sylvester said he had incorporated some suggestions from the defence and the prosecution, but he did not address the defence objections to a narcoanalytic interview and polygraph.

Karen Steinhauser, a former prosecutor who is a law professor at the University of Denver and a defence attorney, said she could not find any case law about use of the narcoanalytic interview. “It comes up so rarely,” she said, adding she knows nothing about it.

She noted the technique is clearly allowed by Colorado law.