WA Opposition leader Mark McGowan has told the Labor Party's state conference that cannabis should be legalised for medicinal purposes.

Mr McGowan said people with terminal or chronic illnesses should be able to access medicinal cannabis in the form of tablets or sprays to ease their pain.

He said he did not support the softening of laws surrounding recreational drug use but would like to give doctors the power to prescribe cannabis when other medications had failed.

"Why should anyone have to suffer in agony if there's another way to relieve their pain?" he said.

"Why should they be treated as criminals?"

Mr McGowan's call comes less than a week after Tasmania's Legislative Council announced it would hold an inquiry into the medicinal use of cannabis.

'Right' to legalise cannabis for those dying in pain: McGowan

Mr McGowan acknowledged his stance would be controversial.

"I believe in this, this is the right thing to do to help those people who are dying in pain or who are in chronic pain," he said.

"We should not run away from tough decisions just because they are controversial."

Mr McGowan said he made the decision after meeting a young man in his Rockingham electorate who was seeking relief through cannabis while suffering a terminal illness.

"He was wasting away in pain and he sought relief through cannabis," he said.

"His family and his carers knew. It was the only thing that helped him cope."