Japanese authorities have raided a factory in Osaka City, western Japan, in an investigation into the widespread health concerns caused by supplements containing "beni-koji" fermented rice.

Sixteen officials from the health ministry and the city entered Kobayashi Pharmaceutical's Yodogawa factory under the Food Sanitation Act on Saturday morning.

The plant was already shut down due to aging degradation, but had produced beni-koji until last December.

The company says the ingredient was made in 2023 at the factory for the product that caused health complaints.

The ministry says the firm has confirmed in its investigation the presence of "puberulic acid," an unintended substance, in a batch of the products. The substance can be produced from blue mold.

The firm says it is not clear if the substance has any effect on human bodies. But the officials are apparently looking into how it was mixed in the products during the production process.

Tomiyama Yuko, chief of Kobayashi Pharmaceutical's investor relations, said the firm intends to sincerely deal with the matter and fully cooperate with the investigation.

The drugmaker said that as of Friday, there were five deaths and 114 reports of hospitalizations among users of the beni-koji supplements. Those affected are said to have developed kidney problems and other health issues.

The company is investigating the link between the supplement and the deaths.