Only about 40 per cent of prescribed end of life drugs in Oregon and California, where they are legal, are actually used, he said.
"They can provide peace of mind and lessen pain and suffering," he said.
"The patient feels involved in choices being made towards the end of his or her life, to the extent that in some situations they have elongated patients' lives.
"Just having the drugs reduced anxiety and can focus someone on quality of life."
Stevens said the charge of assisting someone to commit suicide was rarely brought and difficult to prove.
"The prosecution must establish, beyond a reasonable doubt, that the intention to assist the commission of suicide was there," he said.
"Mrs Austen did not intend that Mrs Treadwell should commit suicide, but assisted her to obtain the pentobarbital. She intended that Mrs Treadwell should have control over her end of life issue - having that [drug] could have had a profound palliative effect to reduce suffering."
Stevens said the two importing charges would be addressed during his closing address.
"I will be making a detailed summary on the unsatisfactory evidence provided by the prosecution on these," he said.
He and Crown lawyer Kate Feltham will make their closing statements on Thursday, and Justice Susan Thomas will provide a summary on Friday, before the jury retires.
- NZN