Thousands of patients are to be given cannabis in the first large-scale study of the drug's clinical effect.
Medical cannabis, which is grown to a precise grade of active ingredient, was legalised in the UK a year ago.
But only a handful of patients have since been prescribed the drug 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: chronic pain, multiple sclerosis, epilepsy, post-traumatic stress disorder, Tourette's syndrome, anxiety disorder and substance abuse.
Professor David Nutt of the organisation Drug Science, which is running the study, 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.
