Warner Chilcott has taken legal action against German drugmaker Schering and its US unit Berlex Laboratories, claiming patent infringement.

The product cites in the lawsuit is the low-dose contraceptive Yaz (drospirenone/ethinylestradiol), which was approved by the US Food and Drug Administration earlier this month and expected to be a big seller for Schering.

Yaz uses a dosage regimen of 24 days of active pills and four of placebo designed to reduce breakthrough bleeding and side effects, but Warner Chilcott maintains this regimen is covered by its patent (No 5,552,394). The company is planning to launch its own product using this regimen - Loestrin 24 Fe - in the USA in April.

“Loestrin 24 Fe embodies a novel new dosing regimen for oral contraceptives and Warner Chilcott intends to vigorously defend our rights under our patent to oral contraceptive products delivering 24 days of active therapy," said Roger Boissonneault, chief executive of Warner Chilcott's parent company.

The company is seeking treble damages, costs and to have Yaz removed from the market.

News of the lawsuit is a blow to Schering, currently heading for a $19.5 billion merger with Bayer, as it expects Yaz to achieve peak sales of around 200 million euros, providing it also wins approval for premenstrual dysphoric disorder, a follow-up indication. The product is part of Schering’s market-leading Yasmin range of oral contraceptives.