Health reporter, BBC News
Uncontrolled asthma can require hospital treatment
Patients with asthma avoid using inhalers or cut down their dose because they cannot afford prescription charges, a study suggests.
Research at the University of Manchester found that patients "skipped" or "stretched out" medicine, potentially making their illness worse.
An Asthma UK survey has also found more than a third of patients are struggling with drug costs due to the recession.
A government review on prescription charge exemptions is due in the autumn.
In the latest study, 30 UK patients who had to pay prescription charges were asked several questions about cost.
In two respondents who were struggling to pay, their asthma was clearly causing them health problems because they were not taking their medicine regularly.
Many of those asked admitted that they would try and make their inhalers last longer or cope without them by avoiding things that would make their asthma worse because of cost, the Primary Care Respiratory Journal reported.
People would choose not to use their preventer inhalers - steroid inhalers used daily to ward off attacks - to save money but were less likely to let themselves run out of the reliever or emergency inhalers.
Dr Ellen Schafheutle who carried out the study said not keeping up with preventer inhaler use would lead to people having unnecessary attacks and needing their emergency inhalers more.
She added that asthma could be very unpredictable and if someone got a cold they could end up needing antibiotics, extra inhalers and oral steroids, each of which they would be charged for.
"What is important to draw out is that cost did not effect everybody in the same way.
"And the other important thing is how essential people view their medication to be.
"There was a group who didn't see their preventer inhalers as essential but the steroid inhaler is the most important for controlling asthma."
Asthma UK found 37% of people with asthma are finding it harder to afford their prescriptions since the start of the recession.
They estimate three quarters of the 73,000 emergency admissions to hospital for asthma every year could be avoided if preventive drugs were taken properly.
Pre-payment certificates are available for people with long-term conditions to save money on prescriptions.
The government has promised to abolish prescription charges for people with long-term conditions but the review into how and when this will happen will now be published in the autumn.
Dr Mike Thomas, a GP and chief medical adviser to Asthma UK, said: "I know of numerous cases where people have stretched or stopped using their preventer medicines to try to reduce prescription charge costs.
"This is often because they notice the immediate effects of relievers more than the potent long-term effects of their preventers.
"It seems illogical that people with some long-term conditions, like thyroid disease, receive free prescriptions, while those with conditions like asthma do not.
"Unfortunately, there isn't much that GPs can do other than increase the number of inhalers on a prescription, which can encourage waste and is discouraged by primary care trusts."