(Reuters) – Online pharmacy Valisure said on Monday its tests showed high levels of a probable cancer-causing impurity in diabetes drug metformin.

High levels of N-nitrosodimethylamine (NDMA) were found in metformin made by 11 companies, including Amneal Pharmaceuticals Inc and Aurobindo Pharma Ltd, the company said in a statement.

Valisure said it had requested the U.S. Food and Drug Administration (FDA) to recall the identified lots of medicines.

The FDA declined to comment, saying it would respond directly to the citizen petition filed by the company.

The agency in December started an investigation into metformin for possible contamination and said earlier this year that it did not recommend a drug recall.

Amneal and Aurobindo Pharma did not immediately respond to a request for comment.

NDMA contamination was responsible for the recall of heartburn drug Zantac sold by Sanofi SA and some generic versions of the treatment last year.

(Reporting by Manas Mishra in Bengaluru and Michael Erman in New York; Editing by Anil D’Silva)