Most Australians with asthma are prescribed too much preventer medicine, which increases the chance of harmful side-effects, experts say.

More than two million people have asthma in Australia but an estimated seven out of 10 over the age of 12 may be getting unnecessarily high preventer doses, according to researchers at the Woolcock Institute of Medical Research.

In an article published in the journal Australian Prescriber, they noted most of the benefit from asthma preventer inhalers was seen with low doses.

But many patients were routinely prescribed high doses of inhaled corticosteroids, leading to a higher risk of adverse events as well as potentially higher out-of-pocket costs.

Lead researcher Helen Reddel said there were many reasons why people might be using a higher dose of asthma preventer medicine than needed.

They included people with frequent symptoms at diagnosis being prescribed high doses without review when their symptoms improved, or patients who experienced a flare-up not returning for review after their preventer doses were increased.

"If your asthma has been stable for more than three months, check with your doctor. It may be possible to reduce the dose," Professor Reddel said.

Asthma affects the airways of the lungs and is one of the three most common long-term health conditions in Australia.

Symptoms include chest tightness, wheezing, coughing and shortness of breath.

Australian asthma guidelines recommend that most adult and adolescent patients should be prescribed low doses of an inhaled corticosteroid or an as-needed combination of low-dose corticosteroid and low-dose formoterol.

Being treated with high doses of medicine over a long period is associated with a small increase in the risk of conditions such as cataracts and osteoporosis.