The drug would be in tablet form
Courses of a common cancer drug can dramatically reduce the risk of a patient with multiple sclerosis having a relapse or deterioration, work shows.
Taking cladribine a few times a year more than halved the chances of a relapse, with few side-effects, the UK study of 1,300 patients found.
UK expert Professor Gavin Giovannoni said the drug could revolutionise the treatment of MS.
Its manufacturer Merck Serono hopes to seek licensing for its use this year.
The drug is already licensed for treating leukaemia.
Prof Giovannoni gave his assessment of its potential value to MS patients at a meeting of the American Academy of Neurology in Seattle.
The UK's drugs watchdog, the National Institute for Health and Clinical Excellence, is considering including cladribine in its next round of assessments.
Cladribine works by suppressing the immune system, reducing the risk of further damage to a patient's nervous system.
Patients who took the drug were 30% less likely to suffer worsening in their disability due to MS.
Easy to take
The study involved over 1,300 MS patients who were followed up for nearly two years and monitored using MRI scans.
Patients were given either two or four treatment courses of cladribine tablets per year, or a placebo.
Each course consists of a single tablet per day for four or five days, adding up to just eight to 20 days of treatment each year.
If it becomes available to patients, cladribine will be the first licensed treatment for MS which does not involve regular injections.
Professor Giovannoni, of Barts and The London School of Medicine and Dentistry, part of Queen Mary, University of London, said: "These results are really exciting. MS can be a very debilitating illness and at the moment treatment options remain limited.
"Having an effective oral therapy will have a major impact for people with MS.
"Our study shows that cladribine tablets prevent relapses and slow down the progression of the disease, making patients feel better.
"Importantly, it does so without the need for constant injections that are associated with unpleasant side-effects.
"We will continue to follow the patients in the trial to see how they fare in the long-term."
Dr Lee Dunster, head of research at the MS Society, said: "These are remarkable results and being able to take a tablet instead of having injections will be a huge step forward for people with MS.
"The evidence is there, but we now need to see cladribine move smoothly through the regulatory process and the price the manufacturer sets will play a crucial part in that."
It is estimated that 85,000 people in the UK currently have MS, with 2,500 new cases diagnosed each year.