Tracy Yeats with her daughter Grace and baby Ellie, Grace's niece, at their home in Carterton.

Cannabis spray provides welcome relief in fight against debilitating disease.

Medicinal cannabis has provided a breakthrough for a teen battling a rare brain illness, but the cost of the unregistered, unfunded drug could prevent the youngster getting long-term treatment.

Carterton's Grace Yeats, 13, was an active, playful youngster until May 2012, when she was hit by severe basal ganglia necrosis, a brain disease that left her cognitively unimpaired, but unable to walk or talk, and wracked by painful spasms.

But she found some much-needed relief when she was approved last month for medicinal cannabis spray Sativex, the only cannabis-based treatment legally available in New Zealand.

Grace's mother Tracy said her daughter's pain, mood and ability to control her movements had noticeably improved, especially when it came to using the iPad communication app she has taught herself to operate.

"Before, she would come up with one word [on the app] and then get stuck. Then [with Sativex] it was just, wow. Communication is just not a problem," said Tracey, who gave up her job as a Plunket nurse to care for her daughter full-time.

Grace is now more cheerful, taking delight in "horsing around" like any other teen, picking objects up and throwing them off the bed with her characteristic cheeky grin.

The 13-year-old is even able to give instructions via her app about her niece Ellie, whom Tracy also cares for.

"She was telling someone how to change her nappy, that goes there, this goes here... just being able to control her arm movements, and her alertness, have improved so much."

A child psychologist who visits Grace regularly to help her deal with her situation had also noticed a big difference, Tracy said.

Grace's pain is severe — recently she cracked a bone in her leg during a particularly violent spasm — but she can't have morphine-based drugs because they clash with some of her other medicines.

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She takes two sprays of Sativex morning and night.

Her prescription allows for an increased dose, which her mother wanted to try in case it brought more improvements. But for now she is holding off because of the $1100-a-month cost.

Because it is not a registered medicine, there is no Government funding.

It was "devastating" to have found something that helped but not be able to afford it, Tracy said.

At the moment, both a doctor and specialist have to apply to the Ministry of Health for a patient to get access to Sativex, with Associate Minister of Health Peter Dunne issuing final approval.

"We hope that some day, Peter Dunne will look at allowing someone to bring in cheaper options from overseas," Tracy said.

"But for now that's the only approved one and they say there's not much uptake, but that's why - it's so expensive."

Associate Health Minister Peter Dunne says Pharmac staff sought clinical advice in August from the Pharmacology and Therapeutics Advisory Committee about using Sativex for multiple sclerosis, pain associated with spasticity and treatment-refractory epilepsy. PTAC recommended funding for all three conditions be declined.

Dunne said Pharmac is the authority on the funding of medicines and while he was sensitive to the distressing circumstances of Grace's illness, it would be inappropriate for him to intervene.

Grace's trial, one-month dose was paid for by the Grace Yeats Trust, set up by friends to raise funds to build a hospital-standard annex so Grace could come home after 16 months in hospital.

But with a year's dose costing more than $12,000, the family is looking at fundraising options, such as a fun run or triathlon.

Grace had also offered to do some paintings for sale, Tracy said.

"That way she can help herself as well, which she's keen to do... but we're open to ideas."

So many people had already done so much for the family, and it was hard accepting help.

"But somehow, some way, I need to continue to raise money for Grace's wellbeing and the Sativex that has proven to be a huge positive in her fight," Tracy said.