A drug used to treat epilepsy can also control debilitating symptoms of multiple sclerosis.
MS patients are given therapy to ease symptoms
Levetiracetam was effective in all 11 patients studied by doctors at Southwestern Medical Center in Dallas, Texas.
Research published in the Archives of Neurology showed the drug also reduced nerve pain.
Multiple sclerosis (MS) experts said the majority of people with the disease experienced muscle spasms and cramps.
Spasms and cramps can be extremely debilitating for those affected, leading to loss of balance, increased risk of falls, pain, fatigue, and walking difficulties.
People who have experienced a stroke or spinal-cord injury may also be affected.
Drugs currently used to treat spasticity may cause memory problems, weakness and lethargy in some patients.
The researchers studied 11 patients who were treated with levetiracetam between January 2001 and June 2002 for MS-related spasticity.
They took the drug for between one and four months, with doses starting at 250 milligrams per day and increasing to 3,000 mg.
It was found that leviteracetam decreased spasticity in patients taking the drug alone as well as in those who took it in combination with other therapies for spasticity.
The drugs also lessened nerve pain, which then improved the patient's mood, so they did not need to take painkillers or antidepressants.
Around 85,000 people in the UK have the condition.
However muscle stiffness did not improve.
Dr Kathleen Hawker, who led the research, said only mild side effects had been observed in patients taking levetiracetam.
She said: "It's amazing how many MS patients can't walk, can't move, and you treat their spasticity and they're fine.
"What's nice about these drugs is that they also work for nerve pain, which in turn improves the patient's mood, so we can use one drug for three things instead of prescribing pain killers and antidepressants in addition to the spasticity therapies.
"We're trying to look at medicines that can be used for multiple symptoms so we don't get into a lot of drug interactions."
Quality of life
Chris Jones, chief executive of the MS Trust, told BBC News Online: "We welcome any study which will improve treatments for people with MS.
"In a recent survey conducted by the MS Trust of 2,265 people with MS in the UK, 82% reported experience of spasticity.
"Of those, 54% said it had moderate or high impact on their quality of life. Figures for pain were similar.
"This study reports promising results. However, we should bear in mind that the sample is very small, and we would need to see these results replicated over a much larger population before reaching any firm conclusions."