WA doctors will be able to legally prescribe medicinal cannabis from Tuesday after the state government threw its support behind changes to Commonwealth laws.

Health Minister John Day on Friday welcomed the new legislation, which will class medicinal cannabis as a controlled prescription drug from November 1.

Dr Andrew Katelaris is one of Australia's biggest medicinal cannabis suppliers. Credit:Rohan Thomson

"Like all controlled drugs, medicinal cannabis will be prescription only, with strict rules around prescribing," he said.

"Medical practitioners will be at the centre of all treatment decisions and patients will need to discuss their situation with their treating doctor.