The US man accused of last year's Colorado movie theatre massacre can be given "truth serum" to prove he's insane if he pleads not guilty by reason of insanity, a judge says.

Suspect James Holmes also could be given a polygraph examination as part of an evaluation to determine if he was legally insane at the time of the July 20 shootings, Judge William Sylvester said.

Holmes, 25, was scheduled to enter a plea 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.

Sylvester approved the use of a "narcoanylitic interview," which refers to 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 pentothol.

The prospect of such interviews that may ensue under such a plea alarmed defence attorneys who filed documents opposing such a 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" drugs.

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. He included both a narcoanlytic interview and polygraph in his order.

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.

- AP