Adding antibiotics to standard drug therapy may slow down the progress of multiple sclerosis, research suggests.
Relapsing-remitting MS can be very unpredictable
Patients showed fewer symptoms and fewer signs of tissue damage when they took the antibiotic doxycycline alongside the MS drug beta interferon.
Louisiana State University researchers believe the antibiotic may block the action of enzyme that destroy certain cells in the nervous system.
Archives of Neurology reports the study involving 15 patients on its website.
However, UK experts warned the study was small, and no comparison was made with patients who did not take doxycycline.
The 15 patients who took part in the study all had relapsing-remitting MS - the most common form of the disease.
Typically, this causes attacks of symptoms such as muscle weakness and spasms, followed by periods of remission.
The attacks result from damage inflicted on the body by its own immune system, which turns in on itself, attacking the nervous tissue.
It is thought that these attacks may be triggered by an inappropriate response to viral or bacterial infections, or another potentially disease-causing agent.
They are certainly very unpredictable, and symptoms come and go, often seemingly randomly.
Many patients with relapsing-remitting MS take the drug interferon, which helps to suppress the immune system, and keep it working more normally.
However, they are still prone to attacks which cause damage to the tissue of the brain.
The study focused on patients who had been taking interferon for at least six months, and who were still experiencing symptoms, and developing new tissue damage in the brain.
For four months the patients took 100mg a day of doxycycline alongside their regular dose of interferon.
At the end of this period brain scans revealed that brain tissue damage was reduced by at least 25% in nine of the patients.
There were also signs that disability levels had improved.
The researchers believe that doxycycline, a member of the tetracycline family of antibiotics, may block an enzyme which destroys nerve cells, thus protecting the brain and increasing the effectiveness of the immune system.
Dr Laura Bell, of the MS Society, said: "Antibiotics are cheap and easily available, which would make them an attractive treatment for MS if they were shown to be beneficial.
"However this study is very early stage in only 15 people with MS and no firm conclusions can be drawn at this stage."
Chris Jones, chief executive of the MS Trust, agreed that the study was small, and had only covered a short period of time.
"A longer trial with more people will be needed before we can properly gauge the value of this combination for people with MS."
Helen Yates, of the MS Resource Centre, said the condition was complex and difficult.
She said other work was examining the possibility that MS was linked to an infection of the bacterium Chlamydia pneumoniae - more commonly associated with respiratory disease - in the brain.
"The growing interest in combination therapies is producing some good results, in particular for those people for whom single therapies have not worked."