The European Commission, which opened the probe on Monday, said it was concerned that Swiss drugmaker Vifor Pharma pursued a misleading marketing campaign, primarily targeting healthcare professionals.

The EU competition enforcer said it has indications of this behaviour going on for many years, which may be in breach of EU rules against companies abusing their dominant position.

"The dissemination of misleading information regarding the safety of Pharmacosmos' iron deficiency treatment, Monofer, may have delayed its uptake. This would ultimately harm patients by stifling competition from an innovative medicine," Commission Vice President Margrethe Vestager said in a statement.

Vifor Pharma said in an emailed statement that the opening of a formal investigation did not mean that any wrongdoing on its part had been established.

"We are fully cooperating with the European Commission during this process and are convinced that we have not engaged in any anti-competitive behaviour," Vifor Pharma said.

Some 1.8 million people are treated with high-dose intravenous iron products for iron deficiency annually in Europe.

Australian biopharmaceutical giant CSL announced a $11.7 billion bid for Vifor Pharma last December, which will give it access to Vifor's treatments for iron deficiency, kidney and cardio-renal diseases, and production sites in Switzerland and Portugal.

(Additional reporting by Marine Strauss in Brussels and Paul Arnold in Zurich; Editing by Bernadette Baum and Susan Fenton)

By Foo Yun Chee