Barr Pharmaceuticals saw its shares dip nearly 5% during trading on Friday after news that the US Patent and Trademark Office declined an application to renew a key patent for its contraceptive pill Seasonale concerned investors.

The company said it now has 30 days to respond to the US PTO’s “non-final rejection,” but that the setback will probably pave the way for rival copycat drugs waiting to enter the market as soon as its patent dies.

Seasonale (levonorgestrel and ethinyl estradiol), which took home sales of $27 million in the last quarter, loses its patent armour on September 5, and generic drugmaker Watson Pharmaceuticals has already sought approval to sell its copycat version as soon as its protection expires.

"While we are disappointed in the determination by the PTO, we intend to fully review all options available to protect our patentable invention, and will respond to the Non-Final Rejection as quickly as possible," commented Bruce Downey, Barr's Chairman and Chief Executive Officer.