GlaxoSmithKline is facing generic competition to a billion dollar-selling allergy spray Flonase in the USA, after the Food and Drug Administration cleared a copycat version of the product from Roxane Laboratories.

Flonase (fluticasone) brought in worldwide sales of £656 million ($1.14bn) last year, £506 million of which came from the US market, but saw its last US patent expire in November of last year.

The FDA approved Roxane Laboratories’ generic version yesterday, and Roxane said it would launch its product straight away.

GSK has argued that it is not a simple matter to make sure that one nasal spray device is as efficient as another, and on that grounds could launch an appeal against the approval.

The FDA does not ordinarily issue press releases on the approval of generic drugs but did so in this case because of the debate about the bioequivalence of nasal sprays. In a statement, it said there is no evidence to suggest that generic nasal sprays are inferior to those of brandname drugs.

"Except for their price, which is much lower, generic drugs are in every way equivalent to their brand name counterparts," said Dr Steven Galson, the director of the FDA's Center for Drug Evaluation and Research.

The agency said it had received several citizen petitions questioning the approval criteria for the drug's bioequivalence and for other aspects of nasal sprays related to the approval, but stood by its criteria.

Roxane Laboratories, a subsidiary of German drugmaker Boehringer Ingelheim, may be the first generic company to win approval for a copycat version of Flonase, but others, including top-ranked Teva Pharmaceutical Industries, are in hot pursuit. Teva’s recently acquired Ivax unit won approval to market a generic version of the drug in Europe last May.