TORONTO -- Health Canada has requested pharmaceutical companies stop distributing ranitidine drugs while it continues to investigate levels of an impurity known as N-nitrosodimethylamine (NDMA) in the drugs.

Apotex Inc., Pro Doc Limitee, Sanis Health Inc., and Sivem Pharmaceuticals ULC are complying with Health Canada’s request and have recalled additional drugs as “they may have been manufactured using an active pharmaceutical ingredient that may contain NDMA above acceptable levels,” Health Canada said in a release.

The affected products from each company can be seen on the Health Canada website.

On Sept. 13, Health Canada announced it was assessing NDMA in ranitidine drugs and was collaborating with the U.S. Food and Drug Agency and the European Medicines Agency.

NDMA is classified as a probable human carcinogen, which means long-term exposure to unsafe levels could increase the risk of cancer, according to a Health Canada release.

Prescription ranitidine drugs are used to treat and prevent ulcers in the stomach and intestines.