While there is not yet a conclusive link between metformin and cancer, the FDA discovered the presence of N-Nitrosodimethylamine (NDMA) in 76 lots of metformin drugs and because of this has issued a voluntary recall order for these metformin products. NDMA is known to cause cancer. It is actually used to cause cancer in rats in laboratory tests. Other drugs such as Zantac have been recalled due to the presence of NDMA and its probable link to many cancers. In lab tests, NDMA was found to consistently cause cancer in mice and is considered very dangerous when high doses are ingested. The FDA and other independent researchers have suggested that 'sustained higher levels of exposure may increase the risk of cancer in humans.'