As officials worldwide continue to investigate problems involving Zantac contamination with cancer-causing impurities, a statement by Australian drug regulators indicate that three-quarters of tablets tested were found to contain high levels of the carcinogen N-nitrosodimethylamine (NDMA).

According to a report by ABC News, Australia’s Therapeutic Goods Administration (TGA) has tested at least 135 different batches of Zantac and generic ranitidine, finding that 75% of the tablets contained more than three parts per million of NDMA, raising health concerns.

Zantac (ranitidine) has been on the market for decades, and is widely used by millions of Americans for treatment of heart burn and acid reflux. However, increasing evidence suggests the active ingredient in Zantac is inherently unstable, and may convert to NDMA when stored at high temperatures, such as in a car, inside the human stomach.

The problem was first reported in the U.S. by the online pharmacy Valisure, which filed a citizen’s petition calling for a Zantac recall in September. Shortly after, the FDA issued a warning about the potential risk of Zantac NDMA contamination, indicating that the agency is continuing to investigate the issue.