The addition of new genes can also cause a plant to remove toxic metals from the soil and concentrate them in the plant tissue. Through a complicated process, on/off switches are controlled to restrict the metal to one section of the plant. These on/off switches for toxic metals are subject to problems and have the ability to contaminate the edible portion of the plant (Union of Concerned Scientists 1999).

In one case, with the intention of producing the Gamma-linoleic acid, some tobacco plants were genetically engineered. Instead of producing the intended acid, it produced toxic octadecatetraenic acid. This was an unexpected and dangerous result, which has the potential to happen with every case of GMOs (Reddy 1996).

In 1999, the British press revealed that GE potatoes, which had been combined with DNA from the snowdrop plant, are poisonous to mammals. These potatoes damaged the immune systems and vital organs of lab rats. Most disturbing however, is the fact that the damage in the rat's stomach linings was likely caused by the CaMv viral promoter, which is spliced into almost all GE foods and crops.