The mother of a boy with one of the rarest types of cancer ever has revealed she gave her son cannabis because she was 'desperate to find something to alleviate his suffering'.

Eastern Daily Press: Deryn Blackwell in hospital. Picture: Supplied.Deryn Blackwell in hospital. Picture: Supplied. (Image: Archant)

Callie Blackwell took the decision to give her then 13-year-old son Callie the Class B drug after he was becoming addicted to the anti-sickness drugs he was being given.

She added that she did not want Deryn - who has fought two types of cancer, one of which has had only 50 cases ever reported - to spend his final days in a 'morphine fog'.

In her new book, The Boy in Seven Billion, written with journalist Karen Hockney, Mrs Blackwell tells the story of her son being diagnosed with leukaemia at 10-years-old.

Eighteen months later the family, who live in Watton, was told he had another cancer - the rare Langerhans cell sarcoma. Only five people in the world currently have it.

Eastern Daily Press: Deryn Blackwell, then aged 14, with his dad, Simon. Picture: Denise BradleyDeryn Blackwell, then aged 14, with his dad, Simon. Picture: Denise Bradley (Image: Archant)

Thought to be the only person to have both conditions at the same time, Deryn is a 'boy in seven billion'.

Serialised in today's Mail on Sunday, Mrs Blackwell makes the revelation that she and her husband Simon decided to give Deryn - who is now 17 and dreams of becoming a chef - cannabis and details the 'frightening' experience of finding it and administrating it to their son.

'Deryn was nauseous and, worse, had become addicted to his anti-sickness drugs,' she said.

'He was allowed a dose every seven to eight hours but within an hour of being given some, he would press the buzzer to call the nurses back in.'

She went on to say: 'We couldn't sit by and watch him spend his last days in a morphine fog. Enough was enough. So I went into the city and purchased a vaporiser pen – specialist equipment for inhaling an illegal drug…'

Mrs Blackwell said she spent hours on the internet researching and 'came across reports of a substance called Bedrocan, a cannabis-based painkiller that wasn't available in the UK. Surely Bedrocan had to be a better option than mind-numbing morphine?'

Mr Blackwell met someone at a garage to get the drug and Mrs Blackwell describes how she discovered what was needed to make the drug suitable for a vaporise pen.

'Deryn, of course, was excited to be trying it with the blessing of his mum and dad, but I felt anxious at the prospect of my son's under-age and illegal drug use, especially as we were in hospital.

'After drawing the curtains so that no one could see through the window, Simon handed the filled pen over to Deryn. We felt like naughty schoolkids who were having a sneaky cigarette around the back of the bike sheds.

'Deryn sucked on the pen, breathed in and blew out a massive cloud of vapour – and we frantically waved our hands around trying to disperse it, although there wasn't the smell of cannabis. It smelt more like popcorn.

'After 10 minutes, Deryn said that the pain had decreased a little and he felt more relaxed – the words we had been longing to hear.'

The cannabis use continued and Mrs Blackwell writes that her son's finger, which had been black and dead, was beginning to heal despite him having 'no immune system and no way of fighting of infection' .

Doctors said they were not sure whether Deryn was dying anymore.

'Then it dawned on me,' she said. 'Only one thing had changed since Deryn started to recover - the cannabis tincture. I couldn't tell the doctors what we'd done.

'I was sure the authorities wouldn't see it the same way as we did but if there was even a minuscule chance that the cannabis tincture was responsible for my son still being alive, I wasn't willing to risk stopping it.'

In 2015, Deryn went back to the Wayland Academy to take his GCSEs. He left school in June 2016 with seven GCSEs.