Mercury is a chemical element that has been known since prehistoric times and has been used over time as a component of paints, in the treatment of various diseases, and later in various industrial compounds. Mercury enters the body transcutaneously, by inhaling or by ingesting food contaminated with mercury. The most common cause of mercury poisoning is eating seafood, but people can get mercury poisoning from industrial processing, thermometers, dental work, and old paints. The most vulnerable are people at occupational risk due to chronic exposure, as well as fetuses in the intrauterine period. Mercury poisoning produces extremely serious effects on the nervous, renal, and gastrointestinal systems. The nature and extent of mercury poisoning depend on factors such as the route of exposure, the rate of exposure, the distribution and biotransformation of mercury in the body, the chemical form of mercury or the mercury compound. Both mercury in the blood and the urine should not exceed 10 μg/L. The critical values of mercury in the blood are >150 μg/L and the lethal ones exceed 800 μg/L. New diagnostic methods allow the rapid and accurate identification of the concentration of mercury in biological products. Treatment is based on the use of chelating agents. However, the prognosis of mercury poisoning is extremely variable, due to the potentially irreversible nature of the lesions.