The phrase "to become toxic" means that something has developed the ability to cause harm or damage, especially when ingested or in contact with the body. Full definition
All substances become toxic when they are in the wrong place in too high a concentration. (wattsupwiththat.com)
Many researchers are seeing sugar as not just «empty calories,» but rather a chemical that becomes toxic in excess. (livescience.com)
Heavy metals: Metallic elements, including those required for plant and animal nutrition, in trace concentration but which become toxic at higher concentrations. (eia.gov)