The European Commission, which acts as the competition watchdog of the 27-nation European Union, said on Friday (3 December) the raids - the latest in a series targeting improper activities in the sector - took place on 30 November in several EU countries.
It did not name the companies involved, but Anglo-Swedish firm AstraZeneca confirmed its premises had been raided and the probe involved its best-selling heartburn and anti-ulcer drug Nexium.
Bayer, GlaxoSmithKline, Sanofi-Aventis, Pfizer, Novo Nordisk and Lundbeck said they were not affected.
French privately-owned drugmaker Servier, which has been involved in previous EU investigations, also said it was unaffected by the latest raids.
"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," it said in a statement.
The inspections were a preliminary step in suspected anti-competitive practices, it added.
"If confirmed, this could be a potential violation of EU antitrust rules that prohibit restrictive business practices," the Commission said.
For AstraZeneca, the investigation into Nexium marks the latest brush with competition authorities in Brussels, who have targeted the company before.
In 2005, the Commission ruled AstraZeneca had breached EU rules by blocking or delaying market access to generic versions of another anti-ulcer drug called Losec, the predecessor to Nexium, between 1993 and 2000.
The company was fined €60 million in the Losec case, although this was later reduced to €52.5 million.
"I can confirm that we have been the subject of inspections and that we are cooperating with the authorities," an AstraZeneca spokeswoman said, adding the inspections related "to alleged practices regarding Nexium in Europe".
The patent protection on Nexium has expired in a number of EU countries but generic competition so far has been limited. Several generic drugmakers launched cheap copies of Nexium in Germany two months ago and AstraZeneca said in October a generic had also gone on sale in Spain.
Nexium was AstraZeneca's biggest-selling drug in 2009, with global sales of $5 billion, although its importance is declining. It was eclipsed this year by cholesterol fighter Crestor, which is now AstraZeneca's most important product.
The European Commission said in October that it planned to review drug patent settlements struck by pharmaceutical firms to delay cheaper generics going to market, as part of its crackdown on illegal deals in the sector.
In a critical report on the sector in July last year, the Commission vowed to pursue drugmakers suspected of anti-competitive acts after an investigation found such delays had cost healthcare providers and consumers billions of euros.
It later started probes into Servier and several firms that make generic drugs and also raided several drugmakers, including Sanofi-Aventis, Novartis and Teva Pharmaceutical Industries.
In January this year, the EU watchdog sought details of deals between originator and generic pharmaceutical companies from July 2008 to December 2009. Its report showed there were fewer anti-competitive agreements.
(EurActiv with Reuters.)