Olivia Newton-John says medicinal marijuana is a key part of her treatment for stage four cancer.

In an exclusive interview with 60 Minutes reporter Liz Hayes, Newton-John says that not only has cannabis assisted with her pain management, sleep and anxiety – but it's having affects on her physical health too.

"I'm incredibly pro cannabis," she told Liz Hayes.

"If I don't take the cannabis, I can feel the pain so I know it's working."

Olivia Newton-John speaks with Liz Hayes. (60 Minutes)

Newton-John was diagnosed with breast cancer in 1992 – and beat it.

But after being free of her breast cancer for nearly 21 years, Newton-John and her family were shocked when it suddenly returned in 2013.

Then in May 2017, she was told the cancer had metastasized and spread to her bones.

Newton-John is maintaining her health with a combination of conventional and alternative medicines and remedies.

But her husband of ten years, John Easterling, says he's confident medicinal cannabis is contributing significantly to maintaining her health.

Easterling, who spent years cultivating herbs from the Amazon, has long held a strong belief in the medicinal power of plants.

In a greenhouse at the Santa Barbara ranch the couple share in California, he grows various strains of cannabis that he uses to help treat his wife.

"Cannabis can be used for so many things," he told Hayes.

"I don't use the word cure…. but I'm confident. We had MRIs showing a lesser number of tumours, and the majority of the other ones are shrinking."

John Easterling (60 Minutes)

John Easterling and Liz Hayes. (60 Minutes)

Easterling points out that Newton-John is also being treated with conventional medicine, but says her health has greatly improved.

"Her mobility is increasing, her energy levels are increasing," he said.

"I can't say it's all cannabis, because we do have a variety of Amazonian botanicals too. Then she's doing an estrogen blocker and another drug that facilitates that estrogen block."

Now a cannabis convert, Newton-John is joining the fight for medicinal cannabis to be legalised. She and Easterling want Australians to have greater access to the plant, like they do in their home state of California – where both medicinal and recreational cannabis is legal.

She's also hoping to break down the stigma surrounding cannabis use.

"It's not a drug, it's a herb and a plant," she told Hayes.

"I think when people use the word drug, it's a misconception as to what it is and it gets people thinking, 'oh it's just another drug', but it's not."

Doctors at the Olivia Newton-John Research Institute will conduct a clinical trial to determine the effectiveness of medicinal cannabis later this year.

Oncologist Professor Jonathan Cebon, (60 Minutes)

Head oncologist Professor Jonathan Cebon, says he and his team want to explore the benefits of medicinal cannabis in the lab.

"There's absolutely no doubt that a lot of patients with cancer use medicinal marijuana and report great benefits from it. So you have to believe what the patients tell you," Dr Cebon told Hayes.

"We're very happy to be able to do a clinical trial with medicinal cannabis to assess its effects on the quality of life with patients and cancer," he said.

The Olivia Newton-John Cancer Wellness & Research Centre's annual fun run will take place on Sunday October 6 2019. For more information on The Wellness Walk and Research Run visit: www.wellnesswalkresearchrun.com.au

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