Inhalers could make asthma worse for some children, it's claimed.
Some medicines for the condition don't work and could even be damaging for children with a certain type of gene.
Professor Somnath Mukhopadhyay, chairman of paediatrics at Brighton and Sussex Medical School, said: "Both asthma reliever and controller medicines may not work well in a proportion of children.
"A simple test can identify those who might benefit from a switch to an alternative, more effective medicine."
The news comes just weeks after it was revealed that exposing newborn babies to germs could help prevent asthma as they grow up.
According to the 'hygiene hypothesis', exposure to bacteria is necessary to prime the immune system early in life.
The findings bolstered the theory that our modern obsession with hygiene and cleanliness has driven a boom in allergies and health problems.
Many scientists believe that modern society, with its mania for sanitation and easy access to antibiotics, makes children hypersensitive to harmless allergens.