The card does not explain why magnesium citrate in particular is used and does not compare it with other forms. Composition per tablet: hawthorn leaves or leaves with flowers 100 mg (6.7 mg as procyanidins) and magnesium citrate 123.75 mg (20.0 mg as magnesium).
General information (not a manufacturer's instruction): at 2–4 units a day this gives about 40–80 mg of magnesium; with kidney disease a doctor's consultation is needed before taking magnesium.