By Adam Brimelow
BBC health correspondent
Motor neurones are found in the brain and spinal cord
British scientists are embarking on a major new trial to assess the impact of the mood stabiliser lithium as a treatment for motor neurone disease.
They say the research is necessary because positive findings from a small-scale Italian study were "too dramatic to ignore".
But they are urging patients with the disease not to take the treatment in advance of their results.
They warn that some side-effects of lithium are potentially dangerous.
There are about 5,000 people in the UK living with motor neurone disease (MND).
At the moment there is no effective cure or treatment. It is often rapidly progressive and always fatal, usually within two to five years.
The disease can affect any adult at any age, although it is more commonly found in men, and is most likely to strike between the ages of 50 and 70.
Lithium, a naturally occuring element, has long been used as a treatment for some forms of depression and bipolar disorder.
But recent laboratory tests and animal trials have suggested that it may also have a protective effect with MND.
The recent trial of 16 people in Italy reported encouraging results.
But the MND Association said the study was small and poorly designed, and that its findings should be treated with caution.
The association's president, Professor Sir Colin Blakemore, said: "If you read the publication optimistically it might be taken to mean that lithium literally cures this disease.
"But it's very important, against the background of patient hopes and expectations, to stand back and ask whether the trial was large enough to make the claims that it did."
The director of the MND Care and Research Centre at King's College London, Professor Nigel Leigh, says patients are asking him every day whether they should be trying lithium, but that only a "tiny minority" are taking it.
"I'm a bit surprised. I thought more would do it.
"I think it's because everybody's discussing this openly on online sites and there's a very balanced discussion, and people are aware that there are side-effects."
These include tremors, stiffness, confusion, kidney damage and harm to the thyroid.
Professor Leigh says the only ethical approach is to do a full clinical trial, where people are randomised "blind", so neither they nor the researchers know if they are taking lithium or a dummy pill.
The 18-month study involving 220 patients who have had MND for between six months and three years will start at 10 centres across the UK.
Patients will be monitored closely for side-effects.
'Bear with us'
Professor Leigh stressed that GPs and patients with MND should wait for the results before taking Lithium.
"We've been here many times before, with drugs that have been promoted as being a fantastic answer. You don't always get the answer you expected.
Safety is paramount. Yes, it's tempting, but please bear with us.
"If you can take part in a trial that's great. We realise otherwise it's patience, and patience can be short in this condition. But it's much safer to wait."
The president of the Royal College of GPs, Professor Steven Field, backed the advice.
He said "While the information is encouraging, it's important to await results of clinical trials because the medicine has serious side-effects which could potentially make some of the symptoms worse."