Medical cannabis cannot be approved for use in children with severe epilepsy on the NHS because there is not enough evidence to prove it helps, a watchdog has ruled.

The decision by the National Institute for Health and Care Excellence (Nice) dismayed campaigners and parents who said their children would suffer as a result.

But they took some hope from a review by NHS England. Like Nice, it said more trials were needed but said the children’s experiences on medicinal cannabis in the UK and abroad should be taken into account as evidence of how well the drug works.

Last year the law was changed so that medicinal cannabis could be prescribed legally after the parents of Alfie Dingley and Billy Caldwell, who both have treatment-resistant epilepsy, publicly defied the authorities to bring cannabis oil into the UK.

But doctors have been reluctant to prescribe cannabis-based medicines in the UK despite the rescheduling of the drug in November because of the lack of clinical trial evidence of its benefits and safety. Nice is calling for more traditional “gold standard” randomised controlled trials, comparing a group of people on medicinal cannabis with a similar group of people on other medication, to answer the crucial questions about safety and efficacy.

At the same time, NHS England published its review of the barriers to access to medicinal cannabis for patients. It agreed more research was needed but, as campaigners have argued, it suggested it should be possible to include evidence from the experiences of children and young adults already taking cannabis-based medicines.

It also recommended a network of children’s doctors to be set up to collect evidence and advice on difficult cases. Many doctors were unwilling to prescribe medicinal cannabis, it said, worried about the possible long-term effects on the brain particularly from longer term exposure to THC, the psychoactive constituent in cannabis responsible for “highs”. THC (delta-9-Tetrahydracannabinol) and CBD (cannabidiol) are the two constituents that have been most investigated for their medicinal value.

Nice also turned down Sativex, a cannabis-based medicine developed for multiple sclerosis, on the grounds that the price was too high. It said the drug was not cost-effective for the NHS and alternatives should be used instead.

Another cannabis-based drug, nabilone, was recommended in the draft guidance as an add-on treatment for adults with chemotherapy-induced nausea and vomiting, but only if other conventional licensed medicines do not work.

Prof Mike Barnes, the chair of the Medical Cannabis Clinicians Society, said Nice was wrong to want to assess medical cannabis on the basis of the same sort of trials used to approve conventional medicines.

Barnes, who is medical director of a chain of private clinics using medicinal cannabis currently advertising to recruit more doctors, said it had “failed to take into account alternative, valid sources of evidence” and he claimed the Nice committee was made up of people with negative views about cannabis. “Those with positive views were excluded from the process,” he said.

On the other hand, he said, NHS England had produced “a positive review that recognises the need for accepting different but valid evidence for the efficacy of cannabis as a medicine”.

Hannah Deacon, whose son Alfie Dingley is one of only two patients with an NHS prescription, said she was extremely disappointed with the Nice decision. “There are hundreds of thousands of people using cannabis-based medicines across the world and it’s having an enormously positive impact on their health,” she said. “Many countries have legalised medical cannabis industries – America, Canada and other G7 countries – they see the evidence is good enough. The UK is insisting on reinventing the wheel for no reason and the people who are suffering are patients.”

Profile Cannabis: a history Show Cannabis: a history The earliest written reference to cannabis comes from China in the third millennium BC, but archaeological evidence suggests that hemp, probably for use in fabrics, was cultivated as early as 8,000BC. Native to Central Asia and India, the plant – known as ganja in Sanskrit – appears to have been recognised for its psychoactive properties among several pre-Christian cultures, some of which used it in rituals. It was banned in parts of the Islamic world in the 14th century, not to mention in some British colonies, but was not widely proscribed across the world until the US took against it. A series of regulations in the US culminated in the Marihuana Tax Act of 1937 that banned possession or transfer of the drug, except for medicinal use. But it was not outlawed for all types of use until 1970, as the establishment reacted to hippie culture – a crackdown that continued with President Nixon’s so-called “war on drugs”, which saw the US place pressure on international governments to follow suit. While countries such as the Netherlands have long defied such pressure, tolerating both medicinal and recreational use, international momentum towards legalisation has only begun to pick up in recent years with relaxation of the law in North America. California kicked off the legalisation of medicinal cannabis in 1996, several states had legalised recreational use by 2012 and a majority of states had legalised medicinal cannabis by 2016. It remains illegal under US federal law. Canada lifted a ban on recreational use in 2018, while the UK legalised medicinal marijuana in the same year, albeit subject to tight restrictions. According to the World Health Organization, there are about 147 million cannabis users worldwide, some 2.5% of the global population. While the US market is exploding thanks to liberalisation of laws at state level, Europe is predicted to be the world’s biggest and most lucrative market for medicinal cannabis. Photograph: PR

She said she was supporting 20 families who were trying to get prescriptions from UK doctors and who could not afford to pay thousands of pounds to travel to Europe every month to get the medicine illegally.

The MS Society also said it was bitterly disappointed. “Nice’s refusal to recommend cannabis for pain and muscles spasms, or to fund Sativex on the NHS, means thousands of people with MS will continue to be denied an effective treatment,” said Genevieve Edwards, the society’s director of external affairs.

“MS is relentless, painful and disabling and yet not a single person with MS has benefited from medicinal cannabis being legalised nine months ago. The government and the companies behind Sativex need to make people with MS a priority. Together with Nice, they must get around the table immediately to make Sativex available. This depends on the manufacturers accepting a lower price for the medicine, as right now the cost is entirely unrealistic.”