See What to Consider When Buying for more about the different forms.
Labels are required to show the amount of elemental magnesium in each serving, but you need to read labels carefully. Some magnesium chelates are also less likely to cause diarrhea but are bulky, requiring larger pills to get the same amount of elemental magnesium. Magnesium chloride is less likely to cause diarrhea and is recommended if you are taking a high dose of magnesium but is best in liquid form rather than as a tablet. (Be aware that, because magnesium chloride tends to attract and hold water, which can cause pills to disintegrate, magnesium chloride pills should be stored in a sealed container, away from other pills - see Storage.) Forms that are more water-soluble, such as magnesium chloride, citrate and many chelate forms (e.g., bisglycinate) may be better absorbed. Magnesium oxide tends to be less expensive than other forms but may also be less well absorbed and more likely to cause diarrhea.
How much magnesium do I need? If you are not getting at least 300 to 400 mg of magnesium from your diet, consider a supplement that will get you to that level.Maintaining adequate magnesium intake also has cardiovascular benefits, and maintaining a proper ratio of magnesium to calcium intake may have cognitive benefits (See What It Does). It also helps control levels of vitamin D and can boost low levels. It may also improve glucose status in people with prediabetes - particularly if they are low in magnesium. In addition, supplementing with magnesium may be helpful for conditions such as migraines and menstrual pain and it is an effective laxative and antacid. What does magnesium do? Inadequate intake of magnesium may modestly elevate blood pressure and increase the risk of osteoporosis and fractures.Conditions that may deplete magnesium include alcohol abuse, diabetes, diseases of the digestive tract, and use of medications such as Nexium and Prilosec. Although magnesium can be easily obtained through the diet and overt magnesium deficiency is not common, many people do not get adequate amounts of magnesium and can benefit from increasing magnesium intake from foods or supplementation. What is magnesium? Magnesium is an essential mineral for proper metabolism and nervous system functioning, including helping to maintain the electrical stability of the heart.