The New South Wales Opposition says an Upper House inquiry into the medical use of marijuana will help the debate to be dominated by evidence, instead of dogma.

A cross-party committee will look at whether marijuana can be used as an effective and safe form of pain relief for sufferers of certain illnesses, such as cancer and AIDS.

Labor's leader in the Upper House Luke Foley, who moved the motion to set up the inquiry, says it will also examine what legal implications surround the medical use of cannabis and how it might be supplied.

"We want to hear from clinicians on up-to-date medical evidence," he said.

"We want to hear from lawyers on the legal issues, and researchers, and get to the bottom of this with evidence rather than dogma."

He says the inquiry's findings should shape state policy on the issue.

"If the evidence comes in that there is a role for medicinal use of cannabis for a certain number of illnesses, I would support reform in that area," he said.

"But lets get the evidence in first - that's why we've formed the inquiry yesterday."

The head of of the Australian Drug Law Reform Foundation says strong measures must be put in place to ensure medicinal cannabis is not used as a gateway to recreational use.

Doctor Alex Wodak says 69 per cent of Australians support the use of medicinal cannabis while 75 per cent support a trial.

He says there is a case for its use in some circumstances.

"It's not a first line drug," he said.

"But, if the first line or second line drug doesn't work for you and you've got terrible symptoms - especially from a terminal condition - then I think we should be able to in a civilised and compassionate society like Australia, we should be able to allow people to use it."

The committee will report back in May.