Tens of thousands of asthmatics may be taking dangerous amounts of steroids, research suggests.
Scientists from Amsterdam University looked at more than 2,300 people with the condition.
They found nearly a third (29 percent) were on high doses of oral steroids, which put them at risk of diabetes and osteoporosis.
Further analysis showed 78 percent of the patients were not taking their steroid inhalers as prescribed.
With their disease poorly controlled, the scientists worry patients are then forced to rely on risky medication.
'Asthma patients using high doses of oral steroids are at risk of serious adverse effects such as diabetes, osteoporosis, and adrenal insufficiency,' lead author Ms Katrien Eger said.
One in 12 adults in the UK are being treated for the common condition, Asthma UK statistics show.
And in the US, one in 13 people suffer, according to the Centers for Disease Control and Prevention.
Many patients rely on steroid inhalers to dampen inflammation in their airways and prevent an asthma attack.
These are considered less harmful than oral versions of the drug because the dose goes directly to the lungs.
However, if symptoms get worse or a patient has an attack, oral steroids may be prescribed.
These help to calm the airways and stop inflammation by blocking the effects of chemicals released by the immune system.
If taken long term, oral steroids have been linked to a higher risk of infection, high blood pressure and weak bones, as well as diabetes.
To better understand how many patients rely on these tablets, the scientists looked at 2,312 asthma sufferers.
'We found 29 percent of asthma patients who were using high doses of inhaled steroids were also taking harmfully high doses of oral steroids of 420 milligrams (0.01 oz) a year or more,' Ms Katrien Eger said.
The analysis also showed 78 percent of the patents did not always take their inhaled medication as prescribed or had an incorrect inhaler technique.
The proportion of asthma patients who do not adhere to their inhaler medication or have poor inhaler technique is likely to be similar in other countries, the researchers said.
Full results will be presented at the European Respiratory Society International Congress in Barcelona.
The scientists stress less-risky 'biologic' drugs are available for those who use their inhaler correctly but still battle symptoms.
'But less than half - 46 percent - are currently receiving them,' Ms Eger said.
'This shows there is potential to substantially reduce oral steroid overuse.'
Biologics target different molecules in the body that contribute to asthma.
For example, Xolair (omalizumab) injections are available on the NHS for people with severe allergic asthma.
This occurs when the body makes chemicals in response to an allergic reaction, which creates inflammation. Xolair blocks these chemicals.
The study did not look into why biologics are not being prescribed.
The scientists speculate patients may be failing to explain their condition to their doctor, with medics also not assessing asthma sufferers thoroughly enough.
'Now there is an increasing number of biologic asthma drugs available that avoid the need for oral steroids, doctors should initiate biologic treatment in suitable patients to reduce exposure to harmful oral steroids,' Ms Eger said.
'Every prescription for oral steroids should alert doctors to assess adherence to inhaled therapies and inhalation techniques in these patients.'
Changing how asthma is managed may also benefit the economy.
'If [doctors] reduce exposure to harmful oral steroids and thus reduce the adverse effects, this could lead to a reduction in the cost of healthcare,' Ms Eger said.
'Another important way to look at this, is patients can exercise more and experience fewer exacerbations of their disease, and so have fewer days off work due to illness.'
Dr Samantha Walker, director of research and policy at Asthma UK, added: 'While long-term oral steroids can be life-saving for many people, for some they can cause toxic side effects such as a diabetes and osteoporosis.
'And our own research shows they can cause weight gain and lead to difficultly falling asleep.
'New biologic drugs can transform the lives of people with severe asthma but tens of thousands of patients in England are unable to access them because they are not being referred for specialist care.
'We are calling for new guidelines so healthcare professionals can confidently refer patients with possible severe asthma.
'In the meantime, GPs should urgently refer patients who have had two or more courses of steroid tablets in the last year who could be eligible for life-saving treatment which could transform their lives.'