Sky News have reported that medical cannabis was legalised in the UK a year ago, but only a handful of patients have had access to the drug on the NHS.
Thousands of patients are to be given cannabis in the first large-scale study of the drug's clinical effect.
Only a handful of patients have been prescribed medical cannabis on the NHS because of what medical authorities have called a "paucity of evidence" that it works and is safe.
The only option for patients is to either source cannabis illegally, and risk prosecution, or pay for a private prescription of the drug.
The new study, called Project Twenty21, will subsidise cannabis for 20,000 patients to test its impact on seven conditions:
post-traumatic stress disorder,
Professor David Nutt of the organisation Drug Science, which is running the study said
"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."
"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."
Professor Wendy Burn, President of the Royal College of Psychiatrists, 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."
Medical cannabis was legalised last November following a series of high profile cases, including eight-year-old Alfie Dingley, whose mother claimed the drug eased his severe epilepsy.