NEW YORK (BLOOMBERG) - High levels of the probable carcinogen NDMA have been found in the US supply of the widely used diabetes drug metformin, according to testing done by the online pharmacy Valisure, which is pressing regulators for recalls.

Valisure's testing found NDMA in 16 different batches of metformin made by 11 drug companies, including Amneal Pharmaceuticals LLC, Ascend Laboratories LLC and Actavis Pharma Inc.

The highest levels detected were in a batch made by Amneal, which had as much as 16 times the acceptable daily limit of NDMA, according to Valisure's findings.

Often used as an initial treatment for individuals with type 2 diabetes to control high blood sugar levels, metformin is the fourth most commonly prescribed drug in the US.

In a citizen's petition submitted to the Food and Drug Administration on Monday, Valisure asked the regulator to request a recall and investigate what happened with the contaminated products.

Peter Cassell, a spokesman for the FDA, said that the regulator will respond directly to Valisure.

Amneal, Ascend and Actavis didn't immediately respond to requests for comment.

In December, the FDA said it was testing samples of metformin sold in the US for NDMA, and that it would recommend recalls of the medication as appropriate. European drug regulators have told companies to test for high levels of the carcinogen in metformin.

Recent testing by the FDA has found low levels of NDMA in some metformin, but in none of those tests were levels found to be higher than the acceptable intake limit of 96 nanograms a day, according to a statement this month.

There were metformin recalls in Singapore late last year and in Canada this month, but so far the US hasn't followed suit.

The FDA has said it will continue to monitor NDMA in metformin and other drug products and provide updates on recalls.

NDMA, or N-Nitrosodimethylamine, has also been found in other drugs, including the blood-pressure medication valsartan and heartburn drug ranitidine, prompting recalls. NDMA can also be found in lower levels in cured and grilled meats, dairy products and vegetables, the FDA has said.

"Everyone is exposed to some level of NDMA. The FDA and the international scientific community do not expect it to cause harm when ingested at low levels," Janet Woodcock, a physician and director of the FDA's Centre for Drug Evaluation and Research, said in December.

It's unclear why Valisure's results differ from the FDA findings. Valisure said it largely followed the FDA's protocol for testing for NDMA in metformin, with the exception of changes it said were made to make the method more precise.

David Light, Valisure's founder and chief executive officer, said it's also possible the FDA got samples that weren't representative of the general drug supply.

Depending on the product, NDMA contamination could have occured due to the manufacturing process, how a drug is stored, or even aspects of the drug itself.

In metformin, Valisure believes the contamination may have originated in manufacturing, but it's possible aspects of the drug itself may have contributed, said Light.

Brenda Breeland, a professional liability specialist who lives in Baton Rouge, Louisiana, has been taking metformin for a decade for insulin resistance. After learning of the FDA's investigation into metformin late last year, she sent in her medication to Valisure for testing. Breeland has since switched to another brand, but still worries about contamination.

"I'm not happy with the fact that I was taking something I thought was going to benefit me, and now I've found out it could be harmful," she said.