TORONTO -- Health Canada has expanded a national recall for certain types of diabetes medications due to concerns that an impurity in the prescription drugs could be linked to cancer.

The recall, issued Wednesday, is for certain brands of drugs containing metformin, which is prescribed to some patients with Type 2 diabetes to help control their blood sugar levels.

The recalled medications contain alarmingly high levels of an organic compound called N-nitrosodimethylamine, or NDMA. The compound is safe when ingested in small doses over a lifetime, but studies have shown that it is potentially carcinogenic above a certain threshold.

The new recall includes six lots of the RAN-Metformin drug, sold in 500 milligram tablets by Ranbaxy Pharmaceuticals Canada Inc. Earlier this month, Health Canada issued a similar recall for eight lots of APO-Metformin ER tablets, sold in 500 milligram tablets by Apotex Inc.

Lot numbers for the recalled products are listed on Health Canada’s website.

Some lots of the recalled drugs contained levels of NDMA that were higher than what is considered acceptable, “if the drug were to be taken over a lifetime,” Health Canada said. Other lots contained NDMA levels that were close to the acceptable limit.

“We are all exposed to low levels of nitrosamines through a variety of foods (such as smoked and cured meats, dairy products and vegetables), drinking water and air pollution. NDMA is not expected to cause harm when ingested at low levels. A person taking a drug that contains NDMA at or below the acceptable level every day for 70 years is not expected to have an increased risk of cancer,” Health Canada said in a statement.

The public health agency advises patients not to stop taking their medication unless they have spoken with their health care provider. Stopping the medication without speaking to a doctor could lead to uncontrolled diabetes, high blood sugars and long-term conditions such as heart disease, nerve problems and even blindness.

The recall comes just a few months after Health Canada announced that it was looking into possible concerns about NDMA in metformin drugs. The federal agency has asked multiple drug companies to test their products, and similar testing was conducted at Health Canada labs.