Magnesium citrate (1:1) (1 magnesium atom per citrate molecule), called below by the common but ambiguous name magnesium citrate (which can also mean magnesium citrate (3:2)), is a magnesium preparation in salt form with citric acid. It is a chemical agent used medicinally as a sative and to completely empty the bowel prior to a major surgery or colonoscopy. It is available without a prescription, both as a generic and under the brand names Citromag and Citroma. It is also used in the pill form as a magnesium dietary supplement. It contains 11.3% magnesium by weight. Compared to magnesium citrate (3:2), it is much more water soluble, less alka contains 29.9% less magnesium by weight. As a food additive, magnesium citrate is used to regulate acidity and is known as E E345. As a magnesium supplement the citrate form is sometimes used because it is believed to be more bio-available than other common pill forms, such as magnesium oxide. However, according to one study, magnesium gluconate is marginally more bio-available than magnesium citrate. Magnesium citrate, as a supplement in pill form, is useful for the prevention of kidney stones.