Magnesium Citrate formula provides 400 mg of Magnesium (as magnesium citrate). Magnesium Citrate offers a high potency of the essential mineral magnesium in its highly absorbable citrate form. Magnesium is essential for calcium metabolism, the production of enzymes and proteins. All Windmill supplements are manufactured in the United States in an FDA-inspected facility whose GMPs (Good Manufacturing Practices) are independently verified by qualified certification organizations. Free of artificial colors, flavors and preservatives, Windmill is a trusted name that has been providing America with quality supplements for over 40 years. Gluten Free