NHS pain specialists have been criticised for discouraging patients in chronic pain from asking to be prescribed legal medical cannabis, with notices put up in Derby Royal hospital warning of serious side effects.

The poster said "in our opinion the risk of serious side effects outweighs any reported benefit", and that "following advice from professional associations the pain consultants and allied clinical staff will not be recommending or prescribing medical cannabis for chronic pain patients".

Despite a number of high-profile cases and a wealth of anecdotal evidence suggesting the efficacy of cannabis in treating conditions resistant to pharmaceutical medicines, health service guidance states "very few people in England are likely to get a prescription for medical cannabis", and that possible side effects include "suicidal thoughts", "hallucinations" and "reduced appetite".

Jon Liebling, political director at the United Patients Alliance, said: “We have seen patients being effectively banned from NHS Trust hospitals and this is yet another example of how much the medical profession have to learn. This flies in the face of the opinion of the chief medical officer Dame Sally Davies, who found there was enough evidence of benefit. And as for the 'serious risks', this shows a serious unwillingness on behalf of some specialists to read the evidence."

He added: "For the estimated 1.1 million medical cannabis patients in the UK, this just isn't good enough. Our NHS, and all doctors, have a duty of care, which many seem to be neglecting at the moment."

VICE spoke to the pain management clinic at Royal Derby hospital, which confirmed "we do not prescribe that at our clinic", and said the poster had been put up after a number of patients asked to be prescribed medical cannabis.

The Royal College of Physicians, which advises the NHS on policy, said a recent analysis showed that, statistically, the risk of harm from medical cannabis is greater than the potential benefit. "We would welcome high-quality studies into the use of cannabis-based medicinal products for pain treatment," a spokesperson added.