Health Canada is warning that several drugs containing an ingredient used to treat high blood pressure and prevent heart attacks and strokes are being recalled because they were found to contain a cancer-causing impurity.

The agency listed 28 impacted products containing valsartan being recalled by their manufacturers.

“An impurity, N-nitrosodimethylamine (NDMA), was found in the valsartan used in these products,” Health Canada said in a news release on Monday. “NDMA is a potential human carcinogen, which means that it could cause cancer with long-term exposure.”

The drugs are manufactured by five companies whose valsartan was supplied by China-based Zhejiang Huahai Pharmaceuticals.

Health Canada said it is monitoring the product recalls, and advises anyone using an affected product to contact their health-care practitioner as soon as possible to discuss treatment options.