SHARES in Glaxo Smith Kline are set to fall sharply tomorrow after the US Food and Drug Administration (FDA) proposed tough new restrictions on the company’s biggest-selling asthma drug.

The FDA said that Advair, known as Seretide in Britain, should be given only to patients who have failed to respond to other treatments.

The restrictions, and proposed changes to the drug’s labelling, could severely hamper sales.

Advair produced revenues of £737m in the third quarter, and its suite of asthma drugs together account for 15% of the company’s total revenue. In late trading in New York on Friday, Glaxo shares fell 4% to close at $49.92.

Glaxo reacted angrily. It said the FDA’s move posed a risk to patients because the class of drugs to which Advair belonged was the most effective treatment for asthma.

“These proposed labelling changes would reserve the most effective asthma treatment until after a patient has failed on other treatment options and therefore may be at risk of severe outcomes, such as exacerbations and potentially death”, said Kathy Rickard, Glaxo’s vice-president for clinical development.

Glaxo pointed out that in July an FDA advisory committee gave a favourable opinion on the drugs, and that no new trial data had been published since then.

A spokesman for the company in Britain said the drug already carried a “black box” label warning of possible side effects, and that the new FDA position on its use would bring it into line with Europe.

“We are surprised and disappointed at the decision,” the spokesman said.

Analysts said the FDA restrictions could have a significant effect on Glaxo’s revenues.

“As this is Glaxo’s lead, highly profitable franchise, news like this is not immaterial, particularly following the stock’s recent share-price run up,” said Timothy Anderson, an analyst at Prudential in New York.

Asthma affects an estimated 15m Americans, one third of them children.

Advair last year racked up sales of $2.8 billion in America alone, making it the eighth-largest selling branded prescription drug in the country. Worlwide sales of the drug in the same period totalled $4.5 billion.

The FDA restriction will apply to two specific Glaxo drugs, Advair Diskus and Severent Diskus, and to a third drug, Foradil Aerolizer, which is co-marketed by Schering-Plough and Novartis.

All three contain chemicals called long-acting beta2- adrenergic agonists, known in the industry as Labas.

“Even though Labas decrease the frequency of asthma episodes, these medicines may make asthma episodes more severe when they occur,” an FDA spokesman said, adding that severe asthma attacks could kill.

The expected fall-out for Glaxo underlines the sensitivity of drug companies to regulatory action in America. Last year the FDA issued a public- health advisory notice on Vioxx, a painkiller and anti- inflammatory drug for arthritis suffers made by Merck.

Merck had told the FDA that there was mounting evidence of an increased risk of heart attacks in patients using the drug.

The American pharmaceutical group’s shares tumbled, and the episode eventually led to the departure of its chairman and chief executive, Raymond Gilmartin.