Thousands of patients are to be given subsidised medicinal cannabis in the UK's first major trial of the drug's effect.

Medicinal cannabis was legalised in Britain last November but patients have found it almost impossible to access it on the NHS.

Medicines derived from the drug are not routinely available on the health service because of concerns not enough research has been done into its benefits.

Patients have had to either source the drug illegally or fork out thousands of pounds a month for a private prescription.

But the new study, named Project Twenty21, will subsidise cannabis for up to 20,000 patients to test its impact on seven conditions.

Medicinal cannabis is to be given to 20,000 British patients in first major trial of drug's clinical effect (file)

They include chronic pain, multiple sclerosis (MS), epilepsy, post-traumatic stress disorder (PTSD), Tourette's, anxiety and drug addiction.

Data will be collected every three months on the drug's efficacy, safety and patient-reported feedback.

Professor David Nutt, from the charity Drug Science, which launched the project on Thursday, said the trial would 'provide a solid clinical database for other medical prescribers to build on'.

He told Sky News: 'I believe cannabis is going to be the most important innovation in medicine for the rest of my life. Cannabis medicines can be life-saving in disorders like severe childhood epilepsy.

HOW IS MEDICAL CANNABIS SAID TO WORK? A broad term for any sort of cannabis-based medicine used to relieve symptoms. Some products that might claim to be medical cannabis, such as CBD oil or hemp oil, are available to buy legally as food supplements from health stores. But there's no guarantee these are of good quality or provide any health benefits. And some cannabis-based products are available on prescription as medicinal cannabis. These are only likely to benefit a very small number of patients. Epidiolex for children and adults with epilepsy Epidiolex is a highly purified liquid containing CBD (cannabidiol). CBD is a chemical substance found in cannabis that has medical benefits. It won't get you high, because it doesn't contain THC, the psychoactive chemical in cannabis. Epidiolex is not yet licensed in the UK but is currently going through the licensing system. In the meantime, the unlicensed medication can be prescribed for patients with Lennox-Gastaut syndrome and Dravet syndrome (both rare forms of epilepsy). Nabilone for chemotherapy patients Many people having chemotherapy will have periods where they feel sick or vomit. Nabilone can be prescribed by a specialist to help relieve these symptoms, but only when other treatments haven't helped or aren't suitable. Nabilone is a medicine, taken as a capsule, that has been developed to act in a similar way to THC. The medicine has been licensed in the UK. This means it has passed strict quality and safety tests, and is proven to have medical benefit. Nabiximols (Sativex) for MS Nabiximols (Sativex) is a cannabis-based medicine that is sprayed into the mouth It is licensed in the UK for people with MS-related muscle spasticity that hasn't got better with other treatments. But its availability on the NHS is limited. The National Institute for Health and Care Excellence (NICE) does not recommend that NHS doctors prescribe Sativex, as it is not cost effective. There is some evidence medical cannabis can help certain types of pain, though this evidence is not yet strong enough to recommend it for pain relief. Advertisement

'There are children who have died in this country in the last couple of years because they haven't had access to cannabis. It's outrageous, it's unnecessary and we want to rectify it.'

Chloe Sakal, director of Project Twenty21, told the Pharmaceutical Journal: 'Medical cannabis has been legal to prescribe for a year now; one of the reasons we’re doing this is to help clinicians feel supported.

'We get requests from doctors every day saying they want to use it with their patients, but they don’t know how to write the prescription.

'We want them to feel like it’s OK [to prescribe medical cannabis] and that they’re supported [to do so].'

She added that Drug Science was 'working hard to make it affordable'. Negotiations remain ongoing as to how much it will cost patients.

Murray Gray, six, suffers from intractable epilepsy which caused him to have more than 600 micro seizures a day.

He spent almost his entire life in hospital and was forced to wear a protective helmet around the clock whenever he was allowed home.

His mother Karen said he was prescribed tens of pharmaceutical drugs, all of which had no effect on reducing his jerks.

But after getting their hands on a private cannabis prescription, he has been seizure-free for 11 weeks and no longer needs his helmet.

But, despite tales like Murray's, there is a great deal of scepticism around the efficacy of medical cannabis.

The clinical watchdog NICE said it should not be prescribed for a range of medical conditions because there is a lack of concrete evidence it works.

But the Twenty21 project has been backed by the the Royal College of Psychiatrists (RCP).

Professor Wendy Burn, president of the RCP, said: 'The College welcomes this pilot project which it hopes will make an important contribution towards addressing the paucity of evidence for the use of cannabis-based medicinal products.

'We hope that this pilot, along with other research such as more much-needed randomised control trials, will continue to build the evidence.'

Tales of families forking out thousands of pounds to pay for private prescriptions and travelling abroad to bring cannabis oils back for their children were what persuaded the Government to legalise them last November.

Cases such as those of Billy Caldwell, 13, and Alfie Dingley, seven, highlighted the benefits of cannabis oil to children with epilepsy who suffer multiple seizures.

But since then, Billy and Alfie remain the only two children to have managed to get a prescription on the NHS.

Children suffering from severe, life-threatening seizures were dealt a huge blow in August when the NHS watchdog ruled against prescribing medicinal cannabis for their conditions.