Hundreds of people in the UK are turning to private clinics for medical cannabis, BBC News has been told.
Since its legalisation in November 2018, there have been very few, if any, prescriptions for medical cannabis containing THC on the NHS.
And this has led some patients, with conditions such as epilepsy and MS, to pay up to £800 a month privately.
The government said it sympathised with families "dealing so courageously with challenging conditions".
Cheryl Keen has been trying to get medical cannabis on the NHS for her daughter Charlotte - who has brain damage and epilepsy - but has been refused twice.
And she had been told it was too expensive and she had not yet tried all the other available options, she said.
"Nothing has happened, nothing has changed [since the legalisation]," Ms Keen told BBC Two's Victoria Derbyshire programme.
"It's absolutely disgusting that anyone is having to pay to go private," she added - something she cannot afford to do.
Campaign groups say by not prescribing cannabis medicines with THC, the NHS is limiting treatment options for patients.
A review earlier this month by NHS England, however, highlighted a lack of evidence about the long-term safety and effectiveness of medical cannabis.
NICE said it was unable to make a recommendation about the use of cannabis-based medicines for severe treatment-resistant epilepsy "because there was a lack of clear evidence that these treatments provide any benefits".
And this has led to the introduction of private clinics.
Grow Biotech, which handles about three-quarters of all medical cannabis imported into the UK, said as of July it had received more than 100 requests for private prescriptions - of which about 60 had been fulfilled.
The new London branch of The Medical Cannabis Clinics has not yet opened but said it had 162 patients on its waiting list - with conditions such as epilepsy, Parkinson's, post-traumatic stress disorder (PTSD) and fibromyalgia.
"Everyone can get an appointment to come here but not everyone leaves with a prescription for cannabis," its director, Prof Mike Barnes, said.
"There are some conditions for which there is good evidence for cannabis to be useful, so you'd have to have one of those conditions - like pain, anxiety, or nausea and sickness in chemotherapy or epilepsy."
The clinic says consultations are carried out to ensure prospective patients have tried all reasonable licensed medication for their conditions and reached "the end of the road for treatment".
Prof Barnes described the service as a "lifeline for patients in need".
Prescriptions cost between £600 and £800 a month but Prof Mike Barnes rejected any suggestion the clinic was exploiting patients.
"This is the only way patients who are in significant need can get access to this medicine," he said.
Government 'failing patients'
A report last month by the Health and Social Care Committee said the hopes of patients and families had been unfairly raised when doctors were allowed to prescribe cannabis.
Labour MP Ben Bradshaw, who sits on the committee, told BBC News the government was "failing patients".
"If anything [since its legalisation], it's become more difficult for people to obtain it," he said.
"[The government] now has to put this right, by delivering on the promises that it's made to the patients."
The Department of Health said in a statement: "To support doctors prescribing these products, we have asked the National Institute of Health and Care Excellence (NICE) to develop additional clinical guidelines and are working with Health Education England to provide additional training.
"The decision to prescribe unlicensed cannabis-based products for medicinal use is a clinical decision for specialist hospital doctors, made with patients and their families, taking into account clinical guidance."