In a small study, they compared the effects of inhaling salbutamol, the drug active in Ventolin - an inhaler used by asthmatics - with a dummy drug. Thirteen mildly asthmatic patients volunteered for the experiment and used the inhaler four times a day for two weeks.
In Britain, more than 3 million people suffer from asthma and the numbers are rising. One person dies from asthma every four hours. The National Asthma Campaign says there is evidence of overuse of the inhalers to relieve symptoms of attacks.
Dr Donald Cockcroft, of the Royal University Hospital, Saskatoon, says in the Lancet the results provide further evidence to 'support detrimental effects of their regular use'.
Researchers found that after frequent use, the amount of allergen needed to promote an attack was cut almost twofold, compared with the patients using dummy drugs. The salbutamol also had less effect on the narrowing of the airway after frequent use.