Multiple sclerosis patients in the UK are to be able to get a cannabis-based pain-relief drug from their doctor for the first time, it has been announced.
The drug is based on cannabis
Sativex has already been licensed for use in Canada to relieve pain in people with MS.
The Home Office has now said the drug can be imported to the UK for individual patient's use.
MS charities welcomed the development as a step towards the drug being fully licensed for use on the NHS.
Eighty-five thousand people in the UK have MS. It is not yet certain how many of them would benefit from Sativex.
The drug is a mouth spray containing two chemicals found in cannabis, tetrahydrocannabinol and cannabidiol.
The announcement is believed to be in response to enquiries to the Home Office from doctors and patients about access to the drug.
'Quality of life'
Under the new arrangements, the prescription of Sativex would only be permitted under Home Office licence.
Sativex can significantly reduce nerve pain in MS patients, a study has shown.
Researchers at Liverpool's Walton Centre for Neurology and Neurosurgery compared the drug with a dummy version in a study of 66 patients.
Dr Carolyn Young, who led the research which was published in the journal Neurology, said the drug was seen to reduce pain and sleep disturbance.
A doctor would have to take responsibility for the prescription of the unlicensed drug, which would have to be imported from Canada for that particular patient.
Primary care trusts could decide to fund the treatment on the NHS. Otherwise, the drug would cost patients approximately £4 a day.
The government has asked a watchdog, the Commission on Human Medicines, to monitor the safety of Sativex.
Mike O'Donovan, chief executive of the MS Society said the prescription of the drug was: "a move in the right direction".
He added: "We believe there is now good evidence that cannabis-derived medicine can relieve distressing symptoms like spasticity [stiffness and muscle spasms] and pain in MS.
"Many people do not find available treatments effective and will now have the opportunity to try a new drug which could significantly improve their quality of life.
"We very much hope it will not be long before it is licensed for NHS prescription."
Chris Jones, chief executive of the MS Trust added: "We must, however, highlight that, as with any treatment to alleviate MS symptoms, those which work for some people may not for others.
"With this in mind, we are pleased that people with MS may now have the opportunity, in discussion with their doctors, to access this treatment safely and legally and find out for themselves whether it is beneficial."
In a statement, the MHRA has said it did not object to the importation of Sativex for use to relieve pain in MS patients, but said it must be informed if the product was intended to be used for any other condition.
It added: "Under current regulations, the MHRA may only refuse an application to import an unlicensed medicine into the UK to meet the needs of a particular patient if there are overriding concerns about the product's safety or quality.
"Lack of proven efficacy is not a ground for refusing the import."
The company which makes Sativex, GW Pharmaceuticals, has been seeking a UK licence for the drug since 2003.
It has been granted a licence to cultivate cannabis for medical research purposes in the UK. Plants are being grown at a secret location in the English countryside.
The company says it will continue to seek a full UK licence for the drug.
Shares GW Pharmaceuticals' have jumped by 20% since news of the Home Office announcement emerged.