TOKYO (AFP) - Japan's health ministry said on Friday it was probing claims that falsified data was used in an Alzheimer's disease study involving major pharmaceutical firms, a day after filing an unrelated criminal complaint against Swiss drugs giant Novartis.

Health officials said they were questioning researchers after being told false data was used in clinical testing for the US$28 million (S$36 million) government-backed Alzheimer's study, aimed at improving diagnosis of the disease.

The research involved 11 drugs firms, including Pfizer and Bristol-Myers Squibb and Japanese giants Takeda Pharmaceutical and Astellas Pharma, medical imaging companies and nearly 40 hospitals and medical organisations. The public and privately-financed study, dubbed J-ADNI, began in 2007.

The allegations came to light just a day after Japanese officials slapped Novartis with a criminal complaint which alleged its local unit exaggerated advertising for popular blood-pressure drug Diovan.

A former Tokyo University professor and project researcher on the Alzheimer's study reported the false data claims to health officials. Novartis was not involved in the study.

"After verifying the facts about these allegations, we will deal with the issue appropriately, setting up an investigation team if necessary," a health ministry official said.

Health Minister Norihisa Tamura told reporters in Tokyo on Friday that the probe would get to the bottom of whether the data was made up and, if so, how it made its way into the high-profile study.

"If there really has been data falsification, that would be a grave problem, so we are investigating carefully," he said.

A report in the Asahi Shimbun Friday said the newspaper had obtained internal documents highlighting at least four instances where researchers linked to the drugs makers and medical institutions tried to falsify data.

In response, a Pfizer spokesman in Japan said the drugs giant supplied some financing for the research, but did not employ any researchers.

Others firms could not immediately be reached for comment.

Health officials lodged the unrelated claims against Novartis months after a university said data in clinical studies might have been skewed to falsely promote Diovan, which is also known as Valsartan, in the prevention of stroke and angina.

There is no suggestion that Diovan is ineffective in combating blood pressure problems.