There have been no new NHS prescriptions for full-extract cannabis oil since the medicinal use of the drug was legalised more than 18 months ago, the Guardian has established.

Wealthier families and those who can successfully raise funds pay about £2,000 a month to access full-leaf cannabis medicines via private prescription for children with rare forms of treatment-resistant epilepsy, while poorer parents are unable to afford the prescriptions.

Experts say that, despite the drug’s legality, rigid prescribing guidelines for doctors set by the British Paediatric Neurological Association – which cite a lack of clinical research and reference disputed theories about the mental health effects of cannabis – make getting hold of the medicine on the NHS difficult in practice.

A freedom of information (FoI) request to the NHS business services authority confirmed there were no new prescriptions for the oil in England between its legalisation and the end of February. There is understood to have been no change since.

On the NHS, full-extract cannabis oil imported from the Netherlands has continued to be prescribed to just two children with rare forms of epilepsy, Alfie Dingley and Sophia Gibson, who both received emergency interim licences from the Home Office in summer 2018 following high-profile campaigning.

“The government’s medical cannabis legislation has been a disaster,” said Hannah Deacon, the mother of eight-year-old Alfie, who used to have up to 500 life-threatening seizures a month but can now ride his bike and go to school – with his seizures hugely reduced.

“If you look at the children who have been given it privately, they are doing extremely well. It’s just terribly cruel and incredibly unfair that we should benefit and other children cannot.”

Deacon has written to every member of the cabinet to express the “abject despondency” parents feel due to the lack of access, after their hopes were raised by legalisation, with thousands of children enduring hundreds of serious seizures a month which it is hoped could be dramatically eased with medicinal cannabis.

She has received just one reply so far, from the foreign secretary, Dominic Raab, who, in a letter addressing her as “Bridget”, began by saying that cannabis use was detrimental to the mental and physical health of communities.

Prof David Nutt, the former government drugs adviser who is behind Twenty21, a large medical registry now providing free full-extract cannabis oil to patients in the UK suffering from a variety of conditions, to address the block on prescribing, said there was “mounting real-world evidence” of the efficacy of medical cannabis.

“I suspect many doctors are afraid of cannabis after years of fear-mongering and just want to be spoon-fed by the pharmaceutical industry,” he said.

With the growing demand from patients unmet by the NHS, there have been at least 313 private prescriptions for unlicensed cannabis-based medicine items, including full-extract oil, since legalisation in November 2018, the FoI data showed.

Full-extract oil contains both THC, the psychoactive part of cannabis that remains illegal for recreational use, and CBD, which is now commonly sold on high streets. Many believe oils including both cannabinoids are most effective in vastly reducing, and even precluding, seizures due to a synergetic so-called entourage effect.

Steve Rolles, senior policy analyst at the Transform Drug Policy Foundation, pointed to Germany, Israel, the Netherlands and Canada, where full-extract oil is available through public healthcare systems, and said the UK system was not designed to deal with multi-component plant-based medicines.

“Unless some kind of bespoke regulatory framework can be found that reduces barriers to access, the more risky scenario of unregulated self-medication with illegally sourced supplies will continue,” he said.

A Department of Health and Social Care spokesperson said: “We sympathise with those patients dealing so courageously with challenging conditions.

“Since the law changed, two cannabis-based medicines have been made available for prescribing on the NHS for patients with multiple sclerosis or hard-to-treat epilepsies, where clinically appropriate. This follows clear demonstrated evidence of their safety, clinical and cost effectiveness. We have also changed how we regulate imports to improve supply and reduce costs.

“However, more evidence is needed to routinely prescribe and fund other treatments on the NHS. We continue to work with the health system, industry and researchers to improve the evidence base and identify what more we can do to minimise the cost of these medicines for patients, NHS or private.”