Many foods and beverages actually contain chemicals that could be toxic if you ate very large quantities. Carrots, for example, contain arsenic. Many plants produce toxins and many spiders, snakes and insects produce venoms that contain powerful toxins. Certain bacteria also produce toxins, e.g., the botulinus toxin found in improperly preserved foods. The mechanisms for toxicity vary widely.
Some chemicals disrupt the body's ability to use oxygen.
In cyanide poisoning, for example, oxygen can get to tissue cells, but cannot be used once it arrives. The botulinus toxin interferes with the transmission of nerve impulses which can lead to paralysis of respiration. Before a toxic effect can occur, there must be exposure. A toxic substance may enter the body through the mouth, lung, or skin. Once ingested, a chemical may be absorbed across the wall of the gastrointestinal tract into the bloodstream.