For now, the Food and Drug Administration said patients can continue taking their medications.
The agency said it has detected a "probable" cancer-causing chemical in the medications, which are sold as both prescription and over-the-counter treatments to treat stomach acid and ulcers.
Zantac maker Sanofi said in a statement it "takes patient safety seriously, and we are committed to working with the FDA."
The FDA is responsible for ensuring that U.S. medicines are manufactured in safe, sanitary conditions. But regulators have struggled for years to inspect the supply chain as pharmaceutical production has spread globally.
For decades the FDA focused its manufacturing inspections on U.S. factories. But over time, most companies have moved their manufacturing overseas to take advantage of cheaper labor and materials. Today, roughly 80% of the ingredients used in U.S. drugs are manufactured abroad, primarily in India and China, according to the Government Accountability Office.
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