The European Commission said Friday that it carried out raids--currently confirmed only by AstraZeneca PLC (AZN)--on the offices of pharmaceutical companies that allegedly delayed market entry of generic drug competitors for a specific medicine.

AstraZeneca, the U.K.'s second-biggest drug maker, confirmed that a number of its sites in Europe had been visited this week, mainly focusing on its top-seller heartburn drug, Nexium, also known as esomeprazole.

"The Commission has reason to believe that the companies concerned may have acted individually or jointly, notably to delay generic entry for a particular medicine," the commission said in a statement.

Friday's announcement is part of a years-long commission effort to crack down on anticompetitive practices in the pharmaceutical sector. The European Union's antitrust body is carrying out various investigations into alleged anti-competitive practices and focuses on pharmaceutical patent settlements that can delay the introduction of generic rivals.

A spokeswoman for the U.K.'s second-biggest drug maker said the raids were focused on AstraZeneca's top-seller heartburn drug, Nexium, also known as esomeprazole.

"We can confirm we are subject of inspections by certain competition authorities which relate to alleged practices regarding esomeprazole in Europe," she said.

"We are cooperating with the authorities," she said, adding that "AstraZeneca takes compliance with all laws seriously and has a fundamental commitment to doing business in an ethical and proper manner."

Nexium was AstraZeneca's biggest-selling drug in 2009, with global sales of $5 billion--but its importance is declining. The drug already faces generic rivals in certain European countries, with the timing of European launches varying from country to country. In the U.S., its most important market, cheap copies of Nexium are not expected until 2014 following settlement deals struck between the UI.K. drug maker and generics firms.

Several generic drugmakers have launched cheap copies of Nexium in Germany. German generics maker Ratiopharm, now part of Israel-based Teva Pharmaceutical Industries Ltd. (TEVA), said in September it would launch its version of the drug in Germany at a discount of more than 60% to the original product.

Neither Ratiopharm nor Teva were immediately available for comment.

Experts say Nexium is 11 times more expensive than generic omeprazole and has minimal if any additional benefits.

-By Alessandro Torello, Dow Jones Newswires; +32 2 741 14 88;