One in seven people will not respond to salmeterol, found in purple or green inhalers, which is given to tens of thousands of children in the UK.
A study of 62 children showed those patients could be identified and given effective treatment.
The results were published in the journal Clinical Science.
Salmeterol, which is found in Seretide and Servent inhalers, is used to relax the airways in the lungs. It is taken by children who cannot control their asthma just with a blue inhaler, which is given to all children when they are diagnosed with asthma.
The reason why some children do not respond to salmeterol is hidden in their genetic code. The drug acts on beta-2 receptors in the airways, however, one in seven people have a genetic mutation resulting in their receptors being a slightly different shape, which the drug struggles to recognise.