Magnesium supplements generally come in the form of magnesium salts bound to another element or amino acid. Some of the main forms of magnesium include magnesium oxide, magnesium hydroxide (the commercial product Milk of Magnesia), magnesium citrate, magnesium lactate, magnesium gluconate, magnesium aspartate, magnesium glycinate, magnesium orotate, magnesium threonate, and magnesium sulfate (Epsom salt).
Some forms of magnesium (such as magnesium hydroxide and magnesium oxide) are typically used for laxative purposes. Magnesium citrate is the most commonly used form of supplemental magnesium because of its high bioavailability and relatively low cost. Magnesium threonate has been investigated for its effects on increasing brain magnesium levels, which may benefit learning and memory,[1][2] although research to date has only been conducted in animal models (rats).
In other words, different forms of magnesium are used for different specific purposes.