A new drug is the "most effective" treatment for relapsing-remitting multiple sclerosis, say UK researchers.
During MS the body's immune system turns on its own nerves causing debilitating muscle problems.
Researchers at the University of Cambridge say a cancer drug, which wipes out and resets the immune system, has better results than other options.
However, there is concern that a drugs company is about to increase the cost of the drug as a result.
Around 100,000 people in the UK have multiple sclerosis. When the condition is diagnosed most will have a form of the disease know as relapsing-remitting MS, in which the symptoms can almost disappear for a time, before suddenly returning.
Built from scratch
The researchers tested a leukaemia drug, alemtuzumab, which had shown benefits for MS in small studies.
In leukaemia, a blood cancer, it controls the excess production of white blood cells. In MS patients, the dose eliminates the immune cells entirely, forcing a new immune system to be built from scratch which should not attack the nerves.
Two trials, published in the Lancet medical journal, compared the effectiveness of alemtuzumab with a first-choice drug, interferon beta-1a.
One compared the effectiveness in patients given the drug after being diagnosed, the other looked at patients given the drug after other treatments had failed.
Both showed the drug was around 50% more effective at preventing relapses and patients had less disability at the end of the study than when they started.
Dr Alasdair Coles, from the University of Cambridge, said: "Although other MS drugs have emerged over the last year, which is certainly good news for patients, none has shown superior effects on disability when compared to interferon except alemtuzumab."
He told the BBC: "It is certainly the most effective MS drug, based on these clinical trials, but this is definitely not a cure."
However, he warned there were side-effects. These include developing other immune disorders.
He said he thought the drug would be most useful for patients for whom standard treatment had failed and in a "minority" of patients as a first-choice drug.
Eventually relapsing-remitting MS can become progressive MS as the good spells become shorter and less frequent. The drug will have no effect on this form of the disease.
The drug has been withdrawn from the market in Europe and the US as the manufacturer, Genzyme, intends to have it licensed as a treatment for MS.
A Lancet editorial warns: "There is concern that with a licence for multiple sclerosis, the cost of alemtuzumab could rise and might become too expensive for many patients and health systems.
"Finding promising treatments such as alemtuzumab is important. But so is keeping alemtuzumab accessible and affordable."
Dr Doug Brown, head of biomedical research at the MS Society, said: "These results are great news for people with relapsing-remitting multiple sclerosis.
"Alemtuzumab has been found to be an effective treatment for people with MS - but it's only useful to them if it's available on the NHS.
"We urge Genzyme to price the treatment responsibly so that if it's licensed, it's deemed cost-effective on the NHS."
The company said it would not come up with a price for the drug "until it is approved by regulatory authorities" and that it would "engage constructively" with the National Institute for Health and Clinical Excellence, which evaluates the cost-effectiveness of drugs for use in the NHS.