Mushrooms are comprised of many cells that are either poisonous by themselves or produce poisonous chemicals as their byproducts. It is upon consumption that the chemicals of the mushroom absorb through our stomach and into our blood streams. While some chemicals are digested easily, some chemical compounds can cause reactions to the stomach, liver or brain of humans. There are hundreds of mushroom types in existence, all evolved to live in different climates, and also a wide array of symptoms. It is the ones that cause bad reactions we consider poisonous. Never eat a mushroom you find outside as they may be poisonous. Mushrooms bought in the grocery store have been grown in closely watched conditions to ensure that the mushrooms are safe to eat.