Nine drug companies have been fined for delaying cheaper generic drugs coming to market.
The European Commission fined the Danish firm Lundbeck and others 146m euros (£125m; $195m) for blocking the supply of cheaper anti-depressants.
Lundbeck said it would appeal the fine and that it "strongly disagrees" with the decision.
It is accused of paying other companies to delay bringing out a generic version of its anti-depressant citalopram.
"It is unacceptable that a company pays off its competitors to stay out of its market and delay the entry of cheaper medicines," said Joaquin Almunia, the European Commission's competition commissioner.
"We are confirming that the so-called 'pay-for-delay' deals constitute severe infringements."
A 2009 report estimated that these practices lead to consumers paying as much as 20% more for their medicines.
"Agreements of this type directly harm patients and national health systems, which are already under tight budgetary constraints," said Mr Almunia.
The Commission also quoted an internal document from Lundbeck referring to forming a "a club" and "a pile of $$$".
The case dates back to 2002.
It is the first fine of this kind in Europe and analysts say it could set a precedent.
"This is the first case of likely many for which we will see fine imposed," said Deutsche Bank analysts Tim Race.