By Kate Kelland

LONDON (Reuters) - Common asthma reliever drugs taken by millions of children around the world may increase the risk of asthma attacks in some patients with a particular genetic make-up, British scientists said on Tuesday.

The researchers found that salbutamol, a popular blue inhaler medicine also known as Ventolin, as well as salmeterol, an ingredient in GlaxoSmithKline's Advair, are less effective in children with a specific gene variant and may in some cases make their asthma worse.

The scientists said their findings suggest that carrying out genetic tests on children before treatment could be a more cost-effective way of treating them.

"This is a global question that needs to be addressed," said Somnath Mukhopadhyay of Brighton and Sussex Medical School.

Salbutamol is called albuterol in the United States and is very widely used, the researchers told a news conference.

U.S. drug regulators have cautioned in the past that asthma drugs like Advair and Serevent, also made by Glaxo, may actually increase asthma risk in some patients. Glaxo said in a statement it had carried out its own studies with Advair and Serevent and found no genetic variation response differences, although the 500 patients in its study were older than 12.

"Albuterol is one of the commonest drugs right across the world. It is used in the United States, in Africa, India..." said Mukhopadhyay. "It's cheap, it's popular, and it's good stuff -- when it works."

Asthma affects more than 300 million people worldwide and is the most common children's chronic illness. Symptoms include wheezing, shortness of breath, coughing and chest tightness.

The British study, which looked at patients aged between 3 and 22 years, showed that asthma patients using their inhaler on a daily basis who carry a gene variant called Arg16 had a 30 percent greater risk of asthma attacks compared with those with the more usual form of the gene.

Those with two copies of the gene showed a 70 percent increase of asthma attacks, but the scientists noted that children taking daily doses were those with more severe asthma.

The study, due to be published by the American Academy of Allergy, Asthma and Immunology, showed the risk is the same with salbutamol and salmeterol, which is longer acting.

The researchers first looked just at salmeterol and reported a possible genetic link to its effectiveness in 2006. They then widened the study to include more patients using salbutamol.

Around 1 million children in Britain have asthma and more than 100,000 carry this gene variant, according to Mukhopadhyay and fellow researcher Colin Palmer of Dundee University.

Mukhopadhyay said the risk may be worse in countries like India, where the Arg16 gene is known to be more common.

"This is possibly a good time to ask the question whether it is cost effective to prescribe all children with the same or very similar type of medicine, or whether we should look at the question of genetic screening," he said.

The researchers said these tests could be simple and relatively cheap, using cheek swabs or saliva tests.

(Additional reporting by Ben Deighton; editing by Simon Jessop)

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