In around 2700 BC, the Chinese emperor Shen Nung, revered as the father of Chinese medicine, ate the flowers of a certain pungent weed. “Medical cannabis. Stop eating. Let go. Eat more, you will see white ghosts walking around,” he wrote in his pharmocopoeia, The Herbal. “Eat long enough, you will know how to talk to the gods.” Shen Nung claimed the plant could treat more than 1,000 ailments, a citation accepted by historians as the first reference to the medicinal qualities of cannabis sativa.

Five millennia later, and medicinal cannabis is the subject of fierce debate in the UK. On Saturday 16 June, the home secretary, Sajid Javid, allowed Charlotte Caldwell from Castlederg, County Tyrone, to resume the only treatment that stops her 12-year-old son, Billy, having daily, life-threatening epileptic seizures: cannabis oil containing illegal levels of the intoxicating compound THC (delta-9 tetrahydrocannabinol) and CBD (cannabidiol), which is a non-psychoactive, legal cannabinoid.

Just days after resuming cannabis treatment, Caldwell says her son’s condition is markedly improved. “He’s progressing well. I’m in awe of him, he keeps bouncing back.”

The case has reignited the debate over medicinal cannabis in the UK, in what campaigners see as an unjustified and callous application of the law against a global backdrop of legal reform. But change is coming fast, thanks to the work of Caldwell, a tireless tornado of a woman.

“People say I’m brave,” she says. “But Billy suffered numerous life-threatening seizures last week. They leave him utterly depleted – unable to lift his arm or even drink from his cup. The stuff I have to endure is nothing compared to that.”

Parents should not have to break the law to keep their children alive

Caldwell began treatment with the oil in the US, where medicinal cannabis is largely legal, in 2016. Her GP, Brendan O’Hare, then prescribed it, and Billy became the first person to receive a prescription for the drug in the UK. A year ago the Home Office ordered O’Hare to stop prescribing it, prompting Caldwell to fly with Billy to Canada to obtain fresh, legal supplies.

On 11 June, Caldwell openly smuggled seven 40ml bottles – a two-month supply – of the oil, made by Canadian firm Tilray, into Heathrow. As it contained 2% THC – banned in the UK in concentrations above 0.3% – customs officials had to confiscate it. Some were said to be in tears as they did.

During the past 19 months of cannabis treatment, Billy’s seizures had stopped. What was a highlight during that period, I ask Charlotte. “A few months back, Billy’s balance improved so much I was able to buy him trainers instead of special-needs boots,” she says. “I was so excited, I got him three pairs. Normal, wee Nike trainers. It was amazing.”

Canadian firm Tilray, which grows medicinal cannabis, supplied the oil for Billy Caldwell. Photograph: Tilray

Following the confiscation of his medicine, Billy began to have seizures again and was admitted to London’s Chelsea and Westminster hospital five days later, sparking a political and, perhaps, moral crisis that was resolved only when Javid granted an individual exemption to the Misuse of Drugs Act. The government has since announced that it will assemble an expert panel to review the law. But time is short: Billy is only allowed the medicine for the next 20 days, and only under hospital supervision.

“MPs say they will assemble an expert panel led by clinicians, and I applaud that,” says Caldwell. “How soon will this happen? When will it produce recommendations and when will they be implemented?”

David Nutt, a former government adviser on drugs, says it is time for a comprehensive shift in policy. “It could – and should – be a turning point,” he says. “His licence is only for 20 days and it must be used in hospital. They will need a new approach to let him out and this must then apply to other cases. Hopefully, we can adopt a policy such as that of Germany, which recognises more than 50 indications for medicinal cannabis, and avoid more lives being threatened, as Billy’s was.”

Billy’s exemption does not mean the law has changed overnight, says Harry Sumnall, a professor in substance use at Liverpool John Moores University. “This does not necessarily pave the way for access to cannabis for others. That would require a comprehensive review of drugs and medicines regulations.”

There are four cannabinoid medicines licensed in the EU, with another, Epidiolex, likely to be licensed in the coming year for treatment-resistant epilepsy syndromes. In the UK, only the THC-containing Sativex oral spray is licensed as a cannabis medicine – for adult MS patients. However, it is rarely prescribed off-label for other conditions, due to its high cost.

Underlying all this is the human endocannabinoid system (ECS), a neuromodulatory system. It consists of three elements: cannabinoid receptors, naturally occurring or “endogenous” cannabinoids (such as anandamide, which is responsible for the “high” experienced by long-distance runners) and the enzymes that synthesise and degrade these endocannabinoids. Cannabinoid receptors are distributed throughout the body, from the gut to the bladder, kidneys, liver, lungs, heart and brain. Sleep, mood, appetite, fertility and libido are affected by the ECS, and cannabis products activate or influence all these processes.

Legal reform, though overdue, is partly hampered by the complexity of the plant, which contains hundreds of chemicals. That means established methods of medicinal regulation and research are hard to apply, creating a chicken-and-egg situation. Until the drug is removed from its place in the schedule 1 list, which asserts that the plant has no medicinal use, testing cannot be carried out easily.

Sumnall is in favour of reform, but says the emerging industry is plagued by non-scientific methods. Is cannabis oil the new snake oil then? Sumnall notes that some sources claim the drug can treat more than 30 wildly varying conditions, from cystitis and depression to Alzheimer’s disease and Parkinson’s. “Despite positive reports of symptom relief from patients, formal medicinal product licensing requires demonstration of safety and efficacy in accordance with scientific norms. This is not to deny that cannabis products might bring relief, but patient experiences are different from the types of evidence needed for medicine development,” he adds.

“There is an element of snake oil here, undoubtedly,” says Gavin Sathianathan of pressure group United Patients Alliance and Forma Holdings, which develops businesses in legal medical cannabis markets around the world. “But that would not be possible if we had well-written, well-enforced regulation.”

Medical cannabis plants being grown near the city of Safed, northern Israel. Photograph: Baz Ratner/Reuters

Steve Moore of thinktank Volteface says reform is urgently required. “Parents should not have to break the law to keep their children alive,” he says. “Now is the time for change.”

Today, that change seems to be approaching. “I have now come to the conclusion that this is the right time to review the scheduling of cannabis,” said Javid on Tuesday, overturning decades of needless, damaging inertia. Dame Sally Davies, the chief medical officer for England, will oversee medical applications to use the drug on a case-by-case basis in a process she said would be more “clinically led”.

Caldwell remains undeterred by all the talk of the complexity of rewriting British drug laws. She says if progress is not rapid enough, she will simply return to Canada and again enter the UK with the medicine that is so demonstrably keeping her son alive. “Will they confiscate it again? I don’t think so, do you?” she says.