Selenium is an essential trace element, mostly contained in food, excessive selenium supplementation will produce a series of adverse effects: 1, neurological symptoms: excessive intake of selenium will lead to acute poisoning, may appear dizziness, headache, drowsiness and other neurological symptoms; 2, digestive system symptoms: excessive intake of selenium will lead to nausea, vomiting, diarrhea and other digestive system symptoms; 3, respiratory system symptoms: excessive intake of selenium will lead to respiratory shortness, sour smell in the breath, severe cases can have bronchitis, bronchitis and other respiratory symptoms; 4, urinary system symptoms: some patients with excessive intake of selenium will lead to poor kidney function and even kidney disease; 5, other: some patients with excessive intake of selenium will lead to hair loss and nail shape changes, children may have growth retardation, coarse hair, and even intellectual changes. Excessive selenium supplementation, serious reactions need to seek medical attention, but the lack of selenium is also detrimental to the body, should be supplemented as needed. Selenium has the effect of improving immunity, as well as scavenging free radicals, lowering blood lipids, delaying aging, and enhancing disease resistance.