A substance for use as a medicament, comprises a solid mixed metal compound of formula (I): M II 1-a M III a O b A n- c .zH 2 O (I) where M II is at least one bivalent metal: M III is at least one trivalent metal; A n- is at least one n-valent anion; 2+a = 2b+£cn; and £cn <; 0.9a. The substance may be made by heating at a temperature of from 200°C to 600°C, preferably from 250°C to 500°C of a substance comprising a compound of formula (II): M II 1-x M III x (OH) 2 A n- y .mH 2 O (II) where M II is at least one bivalent metal; M III is at least one trivalent metal; A n- is at least one n-valent anion; x = £yn 0 <; x ‰¤ 0.5, 0 <; y‰¤ 1 and 0 <;m ‰¤ 10.