Shares in Denmark’s Novo Nordisk tumbled during morning trading in Europe after the firm announced that has become the focus of a US subpoena and a criminal probe into its US marketing and promotional practices.

Novo, a diabetes specialist, said in a statement that it believes the probe “is limited to its insulin products.” It added: “The subpoena indicates that the documents are necessary for the investigation of potential criminal offences relating to healthcare benefit programmes.”

The Danish firm has stressed that it will cooperate with the US Attorney in the investigation, but cannot predict how long the investigation may take.

Shares in the company were dropping on the Copenhagen Stock Exchange during early trading.

It has been a bad month for Novo, and news of this subpoena follows in the wake of US regulators slapping a safety warning on the label of its anti-bleeding drug NovoSeven (recombinant Factor VIIa), which showed an increase in the risk of blood clotting in patients who do not suffer from haemophilia.