Former union boss Helen Kelly uses medicinal cannabis illegally as pain relief for terminal lung cancer.

A group of medicinal cannabis users and supporters say the Ministry of Health was biased in its review of Government guidelines for accessing cannabis products.

Former Council of Trade Unions boss Helen Kelly, Nelson teenager Alex Renton's mother, Rose, and Wairarapa teenager Grace Yeats' mother, Tracy, are part of the group demanding an urgent independent inquiry.

After the guidelines for the application process were reviewed, Associate Minister of Health Peter Dunne last month announced only minor changes, based on the advice of medical experts.

MARION VAN DIJK/FAIRFAX NZ Rose Renton, medicinal cannabis campaigner and mother of deceased Nelson teenager, Alex Renton.

The guidelines were introduced last year after Renton, successfully applied to Dunne for a cannabis product, Elixinol.

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Currently the only cannabis-based product available in New Zealand that didn't require ministerial approval from Dunne was Sativex.

The feedback from the review was "unanimously supportive that the guidelines and process are sound", Dunne said.

His position of a "robust and scientific" approach to cannabis had not changed, he said, which meant "identifying the greatest therapeutic benefits and determining the most appropriate ratios, dosage and delivery mechanisms".

"Otherwise we are essentially flying blind and hoping for the best, an approach that flies in the face of evidence-based medicines policy.

Renton, who suffered from "status epilepticus", a kind of prolonged seizure, died a month after receiving Elixinol.

Sativex provided a breakthrough for Yeats, who was battling a rare brain illness, but the cost of the unregistered, unfunded drug meant the family had to rely on fundraising to cover the cost.

In an open letter the group of medicinal cannabis users and caregivers said the problems with the review included biased methodology, misleading and deceptive scientific claims and a failure to promote patient rights.

"The ministry failed to consult the many patients who the system has failed, those who are using cannabis unlawfully as an effective pain relief, those who have used it illegally overseas, those who suffer in pain through fear of breaking the law, or doctors who understand the benefits cannabis offers.

"No world experts on medical cannabis appear to have been consulted, despite the vast international body of research and expertise," the letter said.

Patients trust their doctors with their lives, the group said.

"Doctors should be free to prescribe medicines that are well-accepted overseas to best suit the circumstances."

"We are willing to share our experiences with the Ministry of Health. The question is whether they are willing to listen," the letter said.