Rust is not toxic in small doses, but can be toxic if a large quantity is ingested or if it enters the body intravenously. Any doses greater than 100 to 200 milligrams per kilogram could be considered lethal.
Iron is an important mineral needed for healthy blood. Iron needs to be ionized to be absorbed into the body, but oxidized iron (rust) has already gone through the chemical reaction. It is not as easily absorbed by the body. Problems arise when there is a large amount or if iron is absorbed in the intestinal tract easily. Vomiting, diarrhea and gastric bleeding are all signs of toxicity from iron and rust. If it progresses beyond this stage, it can lead to shock, hypotension, cardiovascular collapse and possible death.