Shares in UK-headquartered pharmaceutical company GlaxoSmithKline fell 5% yesterday as investors examined the impact of a US move to restrict the use of asthma drugs based on its long-acting beta agonist salmeterol.

Contributing to the fall-off were analysts reports reining in sales projections for GSK’s top-selling asthma product, Advair (salmeterol plus fluticasone) as a result of the action by the US Food and Drug Administration (FDA).

On Friday, the agency said that Advair, GSK’s Serevent and Foradil (formoterol), a LABA sold in the USA by Schering-Plough, should not be used as first-line treatment in asthmatics because they had been linked to more serious breakthrough asthma attacks, despite being designed to provide background control of symptoms throughout the day.

This shift in usage from first- to second- or third-line treatment could restrict Advair’s sales growth, said analysts at Credit Suisse First Boston, who downgraded sales forecasts for Advair from £2.93 billion to £2.25 billion in 2010.

However, others said that Advair has been growing fast despite a black box warning pointing out the risk of more severe breakthrough attacks, which were uncovered in the SMART study of the drug. The impact of the FDA’s advisory on Advair’s growth would be limited, particularly in the USA where it has no direct competitor, and would mainly affect new prescriptions.

Advair, marketed as Seretide outside the USA, is GSK's biggest selling drug; it brought in £737 million to the company's coffers in the third quarter of this year, or around 15% of GSK's total product turnover, while Serevent added another £79 million.

In Europe Advair competes with Symbicort (formoterol and budesonide), sold by AstraZeneca.