Japan's health ministry says blue mold likely got mixed in with supplements containing "beni-koji" ingredients, or red yeast fermented rice, during production at the factories of Osaka-based drugmaker Kobayashi Pharmaceutical.

The supplements have been linked to numerous reports of kidney problems.

On Tuesday, the ministry and the National Institute of Health Sciences released the interim results of the analysis of the samples of the beni-koji material produced between last June and August.

They said they confirmed the presence of blue mold in the samples from the company's related plants in Osaka and Wakayama prefectures.

After cultivation, they found that the mold generated puberulic acid, which had been detected in the products that caused health problems.

They also said the two compounds found earlier in the samples of the beni-koji ingredient linked to health complaints were likely generated by a mixture of blue mold and "monacolin K" substance produced by beni-koji.

The ministry and the national institute officials explained that blue mold at the factories were likely mixed in with the product and generated puberulic acid and other compounds.

They also said tests using mice confirmed that puberulic acid is harmful to kidneys.