Magnesium 400 Citrate by Gsn is a supplement in tablet form designed to provide magnesium citrate, a highly bioavailable form for better absorption. Each tablet contains 672 mg of anhydrous citrate and 128 mg of magnesium, essential nutrients for the body. Proper magnesium intake can contribute to energy generation and bone and muscle development. A magnesium deficiency can cause symptoms like fatigue and weakness.