Union boss Helen Kelly's call for better access to medicinal marijuana has been backed by MPs from both sides of the House.

Ms Kelly, who is terminally ill, has admitted to using cannabis oil for pain relief and wants Government to improve access to the drug.

She is standing down from her position as head of the Council of Trade Unions this week because of her lung cancer.

Prime Minister John Key said yesterday[MON] that Government was not considering a law change.

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"There's a process that people can go through and as you see that process works.

"We work on an evidence and science-based approach and ... if someone wants to bring up a particular case for use to the minister's office in good faith he will look at it."

At present, patients seeking medicinal marijuana must apply directly to the Associate Health Minister Peter Dunne.

MPs gave a broad range of views on the issue at Parliament this morning, with many saying they would vote in support of improved access to medicinal cannabis if a conscience vote was held.

Senior Citizens Minister Maggie Barry said there was a case for people to get access to new forms of palliative relief.

"Morphine is a very good drug ... but I know that there's some pain that morphine cannot reach."

National MP Chester Borrows said he would definitely vote to decriminalise medical cannabis.

Labour's deputy leader and former Health Minister Annette King said the process for accessing medicinal marijuana was too convoluted.

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She said it should be made available for prescription by an oncologist or GP and it should be subsidised by bulk-buying agency Pharmac.

"It has been shown to be very useful for people who have spasms and so on. [But] it's very difficult to get now. The only medicinal cannabis that's available is not subsidised by Pharmac and we did raise those issues at the select committee some months ago."

Some MPs said existing medicines were much more harmful than marijuana.

Labour's associate health spokesman Iain Lees-Galloway said opiates used for pain relief were far more addictive when used as recreational drugs than cannabis.

Labour MPs Kelvin Davis, Phil Goff, and Phil Twyford all said they would probably support a law change, while others including David Shearer said the law should at least be reviewed.

A number of senior MPs said they would vote against a law change if a conscience vote was held.

Social Development Minister said she was firmly against decriminalisation, even if it was purely for medical purposes.

"I would absolutely be anti any loosening of our cannabis laws. Children's lives are completely destroyed by their parents' use of cannabis.

"There may well be people who want to use it in a very measured and responsible way. Unfortunately there are too many people that use it and destroy the lives of others."

Government Ministers Gerry Brownlee and Paul Goldsmith said they would probably oppose a law change, while Education Minister Hekia Parata said she had not considered the issue.

Labour's West Coast MP Damien O'Connor is drafting a bill private member's bill which would improve access to cannabidiol.

He started work on the bill after the death of a Nelson teenager Alex Renton, who had taken a hemp-derived treatment for repeated seizures.