As many as 100,000 children in the UK who are living with asthma might not be getting as much help as they thought from their inhalers.
Scientists have found the medicine inside certain inhalers isn't very effective for some children.
That's because they have something in their bodies which means the drugs in "blue" inhalers don't work as well.
The inhalers do still make it easier to live with having asthma, but not as much as they do with other children.
The researchers looked at 1,000 children with asthma and found that those who carried two certain genes were more likely to suffer an attack if they needed their inhalers every day.
Using an inhaler still made it more comfortable for them, but not as much as for children without the genes.
Keep using inhalers
The people behind the study reckon the results are really important, but say kids with asthma should not change the way they use their inhalers until more research is done.
If you are worried about your asthma medicine then chat to your parents or a teacher about what you should do.