Generic copies of GlaxoSmithKline’s antidepressant Paxil (paroxetine) have been given leave to remain on the US market, after a Supreme Court ruling backed an earlier verdict that invalidated part of GSK’s patent on the drug.

The UK-based drugmaker was trying to overturn a lower court decision that invalidated one of its patents and allowed Canadian generics firm Apotex to enter the marketplace for paroxetine in 2003.

Although Paxil's US composition patent did not expire until this year, generics companies have been developing anhydrate forms of the active compound in Paxil, which they say do not infringe either GSK's hemihydrate formulation or its anhydrate patents.

Sales of Paxil slumped after the introduction of Apotex’ generic. While it was once a $3 billion product, the product has shrunk rapidly to a turnover of around $1.1 billion last year.