The study suggested symptoms of asthma were reduced by the drug
Antibiotics could be useful in the treatment of asthma, research suggests.
There is no cure for the condition which affects 5.2m people in the UK but its symptoms are eased using steroids.
But an international study of 278 patients suggests an antibiotic, Telithromycin, can reduce symptoms of asthma and improve lung function.
Recovery times from acute attacks were also reduced by the drug, which is used to treat sinusitis, the New England Journal of Medicine study said.
This development could open up a whole new area of research
Professor Sebastian Johnston
Patients in 70 centres around the world, including St Mary's Hospital in London, were enrolled in the study within 24 hours of needing medical attention for an acute asthma attack and split into two groups.
One group were given 800 mgs of Telithromycin daily for 10 days while the other received a dummy version of the drug.
Both were given the usual treatment for asthma as well.
Symptoms and lung function for the patients in the Telithromycin group improved significantly compared to those given the dummy drug.
Improvements were around twice as great towards the end of the treatment period, the researchers said.
And recovery time from an acute attack was also reduced from eight to five days for those on the drug, which is not licensed for the treatment of asthma.
However, no improvement was seen in tests of lung capacity.
Most acute asthma attacks are thought to be linked with viral infections which are not affected by antibiotics.
But the researchers believe the positive effects of the drug may be as a result of its impact on two bacteria - Chlamydophila pneumoniae and Mycoplasma pneumoniae - found present in 61% of patients in the study.
They think the presence of these bacteria could make asthma attacks more severe and that the anti-inflammatory effects of Telithromycin play a part in reducing recovery time.
Research leader Professor Sebastian Johnston, from Imperial College London, said that although antibiotics traditionally have not proven effective in treating asthma attacks, this development could open up a whole new area of research.
"Although we're not sure about the exact mechanism which caused this antibiotic to be effective, this study indicates it does clearly have a beneficial effect.
"We still need further trials to confirm these results, to investigate the mechanisms of action of this treatment, to see if the same benefits are seen with other related antibiotics and to see which patients are most likely to benefit."
A spokeswoman for Asthma UK said: "This is an interesting first step into a possible new approach to asthma.
"Further research will be needed before we can fully determine the possible benefits for people with asthma."
Telithromycin is not commonly used in the UK but can treat a range of infections such as community-acquired pneumonia, chronic bronchitis and sinusitis.