Health insurance group Menzis is taking legal action against Astra Zeneca, accusing the pharmaceuticals firm of ‘dragging out’ patents to make it pay too much for drugs.

Menzis wants €4m in compensation for its clients, whom, it says, paid too much for a drug named Seroquel. The case will be heard by judges in The Hague on Friday.

Seroquel is a drug which helps people who suffer from schizophrenia, bipolar disorder, and major depressive disorders. The patent on one variety expired in 2012 but Menzis says Astra Zeneca prolonged it until 2014 by a process known as ‘evergreening‘.

Once the patent expired, the price of the drug plunged 90% as other generic drugs firms began making their versions.

‘It is the first time that a health insurance has taken a drugs company to court for this kind of thing,’ Henk Eleveld, a pharmacist who advises Menzis, said in a statement.

‘We are taking this step because of ethics. Pharmaceuticals companies keep the price of medicine artificially high and it is people who pay for health insurance who pick up the bill.’