Novartis' marketing practices in Japan are under the spotlight after the country's health ministry called for a criminal investigation into the way the Swiss major has promoted its blood pressure blockbuster Diovan.

The ministry has filed a formal criminal complaint with the Tokyo public prosecutor’s office in response to alleged misleading advertising. The problem stems from data manipulation of a post-marketing trial of Diovan (valsartan) conducted by a team at Kyoto Prefectural University of Medicine led by a former professor whose published papers on valsartan were withdrawn from medical journals after questions were raised over the validity of the findings.

The study also involved statistical analysis by a former Novartis employee who took part in other trials of valsartan at Japanese universities – all without disclosing any affiliation with the company.

Now, according to a New York Times report, Jiro Akagawa, director of the Health Ministry’s Compliance and Narcotics Division, has told reporters that a criminal probe was sought because the ministry's own investigation, based on materials submitted by Novartis, has failed to shed light on who was responsible for altering the data.

Novartis has previously admitted improper behaviour by an unnamed employee who was involved in the studies, but denies that the company knew anything about the disputed data.

In a statement on its Japanese website, according to the Financial Times, Novartis said it took the ministry’s action “extremely seriously” and promised to co-operate with authorities without admitting any wrongdoing. The statement goes on to state that "we apologise deeply to patients, families, medical practitioners and the Japanese people for causing great nuisance".

Diovan is still a big earner for Novartis ($835 million in the third-quarter) although they are in decline following the loss of patent protection in all the key markets.