Pfizer (NYSE:PFE) has stopped international shipments of the smoking-cessation drug Chantix (varenicline) after concluding it had elevated levels of a potential carcinogen.

The substance in question, nitrosamines, are organic compounds thought to cause cancer in various organisms.

“We believe the benefits of Chantix/Champix outweigh the very low potential risks, if any, posed by nitrosamine exposure on top of other common sources over a lifetime,” said Pfizer spokesperson Steven Danehy. Champix is the brand name of Chantix outside of the U.S. “We are actively working with regulatory authorities around the world to communicate the appropriate information in accordance with specific actions and timing, which vary by country.”

Pfizer suspended distribution as a precaution. The company is also researching to determine whether additional lots of the drug contain nitrosamines.

STAT News first reported the news.

Earlier this month, health officials in Canada announced a recall of the drug known as Champix internationally. An announcement posted on a Canadian government website cited the presence of the “impurity N-nitrosovarenicline above the acceptable concentration limit” in an affected lot of the drug.

Nitrosamines are also present in various foods, including beer, cured and grilled meats, some vegetables and dairy products. Nitrosamines are also found in some drinking water.

FDA approved the smoking-cessation drug in 2006. The agency has indicated the medicine for a 12-week period but allows for an additional 12 weeks of therapy if needed.

Health authorities have not identified a cancer-related safety issue related to Chantix/Champix, yet continued smoking can lead to a tangible increase in cancer risk.

FDA is continuing to test a range of medicines for nitrosamines, which scientists have also identified in hypertension medications, leading to a series of recalls. Manufacturers across the pharma industry have also had ongoing programs to identify the presence or formation of nitrosamines in drugs.

Health authorities worldwide have identified acceptable daily limits for nitrosamines in medicines to maintain a theoretical excess cancer risk of 1 in 100,000 or lower. For context, between one in two and one in three people in many parts of the world will develop cancer at some point in their lives.