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I Heard Metformin Might Contain a Cancer-Causing Chemical. Is It Safe to Take?

Craig Williams, PharmD, responds:

There have not been any U.S. recalls of metformin due to concerns about toxic impurities. If it has been prescribed for you, you should continue to take it.

What To Know

Metformin is an oral blood glucose–lowering medication used by people with type 2 diabetes. It’s often the first drug doctors prescribe for diabetes management, alongside lifestyle changes.

In January of 2020, the Food and Drug Administration (FDA) announced that some forms of metformin in other countries were reported to have low levels of NDMA (short for N-nitrosodimethylamine), an environmental contaminant. According to a statement from Janet Woodcock, MD, director of the FDA’s Center for Drug Evaluation and Research, those levels are “within the range that is naturally occurring in some foods and in water.” While NDMA has been classified as a probable human carcinogen (a substance that could cause cancer), it’s considered safe for human consumption at low levels—96 nanograms or less per day. For comparison, 1 nanogram is 1 billionth of a gram. A grain of salt, which weighs about 58,000 nanograms, is 600 times more than the safe allowable limit of NDMA. 

Dozens of different manufacturers make and supply metformin for the U.S. market. Most of those companies are international and sell metformin all over the world. Any company that sells drugs in the United States is subject to investigation by the FDA, even if its drugs are produced in another country. While many groups have been critical of the FDA for not conducting more international inspections, the FDA has not found unacceptable levels of NDMA in any formulations of metformin sold in the United States. As a result, there currently are no U.S. recalls of metformin.

Find Out More

The FDA regularly posts information regarding drug safety and availability on its website (fda.gov/drugs/drug-safety-and-availability). The site is written in an accessible format and is intended to be used by consumers as well as clinicians and manufacturers. The FDA also maintains a toll-free phone number to answer consumer questions at 888-INFO-FDA. For more information on nitrosamines and NDMA in particular, visit the World Health Organization’s website: who.int/medicines/publications/drugalerts/InformationNote_Nitrosamine-impurities.

Takeaway

Metformin was approved for use in the United States in 1995, and it remains a safe and effective first-line drug for many people with type 2 diabetes. The FDA continues to monitor the supply of metformin in this country and hasn’t found any evidence of toxic impurities. If you’re taking metformin, continue to do so until your doctor advises otherwise.

Craig Williams, PharmD, is a professor of pharmacy at Oregon Health and Science University Medical Center in Portland, Oregon, and an associate editor of Diabetes Forecast.

 

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