An alkaloid is a nitrogenous organic molecule that has a pharmacological effect on humans and other animals. Alkaloids are found in plants (e.g., in potatos and tomatoes), animals (e.g., in shellfish) and fungi. Some are produces in the human body (e.g., histamine).
Even though many alkaloids are poisonous (such as strychnine, coniine, and colchicine), some are used in medicine as analgesics (pain relievers) or anaestheticss. Morphine, cocaine, atropine, caffeine, quinine, theophylline, and theobromine are also members of the alkaloid family.