Scientists have called for further research into the effect of cannabis compounds on cancer cells after a teenage boy who was given the drug by his mother survived the disease.

Callie Blackwell said she decided to give cannabis to her son Deryn, who was suffering from a rare, aggressive form of leukaemia, to ease his pain and anxiety as he lay dying in a hospice.

After unsuccessfully requesting a prescription for a cannabis-based painkiller from a doctor, Ms Blackwell and her husband Simon met a dealer in a service station to buy some cannabis, which they prepared at home in a pressure cooker using instructions found online.

“I thought: ‘what have I got to lose? He’s dying anyway’. The effects of it blew my mind. It wasn’t what I expected,” Ms Blackwell told ITV’s This Morning.

Ms Blackwell said she expected 14-year-old Deryn, who had undergone multiple rounds of chemotherapy and radiotherapy following his diagnosis at the age of 10, to die when doctors said nothing else could be done.

But Deryn, who is now 17, made a gradual recovery and is now studying catering and has a part-time job as a vegan chef.

Cancer experts have warned stories like Deryn’s cannot prove the efficacy of one treatment over another until properly controlled clinical trials have taken place.

“There have been lots of studies looking at the effect of cannabis on cells growing in the lab, but that’s been quite mixed, it seems to have had different effects on different types of cancer cells,” said Emma Smith, science information manager for Cancer Research UK.

Dr Smith told The Independent Deryn’s recovery was “wonderful news”, but said: “It could have been a number of things. Perhaps cannabis did help, perhaps it didn’t. 

“Because it’s just one person’s story, without a doctor analysing all the clinical evidence and comparing him to somebody that didn’t get cannabis, we still don’t know for certain it was the cannabis that helped him.”

Wai Liu, a senior cancer research fellow at St George’s University of London, has led research into the potential anti-cancer properties of chemicals found in cannabis such as cannabidiol (CBD) and tetrahydrocannabinol (THC).

“I try and separate the science from clinical studies from anecdotal evidence, but there are certain compounds in cannabis, namely CBD and THC, which in a laboratory are anti-cancer in effect,” he said. 

“There’s no two ways about it. What it does to certain cancer cells is precisely the same thing as drug companies are trying to develop. 

“But the difficulty is always translating what we see in clinical and animal studies into what we see in humans.”

Dr Liu told The Independent he often received reports based on anecdotal evidence of people who self-prescribe cannabis.

“Some patients are getting the benefit, but I don’t know if it’s due to the drug or something else, because it’s not controlled,” he said.

“There’s something in it worth exploring, and that’s what a number of scientists are trying to do.”

Peter McCormick, a cellular neuroscience and biology researcher at the University of Surrey, said Deryn’s case was “very heartening” and “validates that we need more research into what is going on”.

“There are such stories, not his but others, where benefits have been seen. There are others where it hasn’t worked, so the bottom line is we need more research to understand what’s in play here.”

Oxford University recently announced a new £10m research programme into the medical use of marijuana. Scientists will explore the potential benefits of cannabis compounds in an attempt to create new treatments for conditions including pain, cancer and inflammatory diseases.

Zameel Cader, associate professor in clinical neurosciences, said the medical use of marijuana was an “area of huge untapped potential” – but the Home Office has said there are no plans to make the “harmful drug” legal.

The mouth spray, called Sativex, contains two chemical extracts taken from the cannabis plant.

It was licenced for use in the UK in 2010, but is not usually available on the NHS in England as it is deemed too expensive. It is, however, available to MS patients in Wales.

A clinical trial into Sativex last month showed improvement in patients with a certain form of brain cancer when combined with another drug.

“It is a positive result, we’ll have to wait for bigger trials to be carried out, but it does suggest that at least for brain tumours, there is some promise of benefit to treating people with a cannabis extract, in this case Sativex,” said Dr Smith.