Till now, there is no effective cure or treatment for the fatal motor neuron disease, which is commonly found in men and most likely to strike between the ages of 50 and 70.
However, it can affect any adult at any age.
An Italian study had shown that lithium might have a protective effect with MND, but the British scientists said that the findings should be treated with caution.
The researchers said that the study was necessary because positive findings from a small-scale Italian study were "too dramatic too ignore".
They warn that some side effects of lithium are potentially dangerous.
The director of the MND Care and Research Centre at King's College London, Professor Nigel Leigh, said that the patients were asking him every day whether they should be trying lithium, but that only a "tiny minority" were taking it.
"I'm a bit surprised. I thought more would do it," the BBC quoted Leigh as saying.
"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."
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," Leigh said.
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.