An antacid preparation comprises a non-toxic antacid substance which is an oxide, hydroxide, carbonate or hydroxy salt of magnesium together with a non-toxic water-soluble synthetic polymer which is a polymer of acrylic acid cross-linked with a polyhydroxy compound having at least 3 hydroxy groups, such as a polyhydroxy alcohol, wherein the hydrogen atoms of at least three hydroxyl groups are replaced with unsaturated aliphatic radicals, preferably having 2-4 carbon atoms, or cross-linked with an unsaturated ether or unsaturated saccharide ether or unsaturated ester having at least three double bonds available for cross-linking such as the tri-acrylic acid ester of glycerol and acrylic acid esters of glycerol having from 3-8 acid residues. The polyhydroxy compounds specified are glucose, fructose, mannose, galactose, sucrose, lactose, maltose, glycerol, erythritol, dulcitol, mannitol, sorbitol, pentaerythritol.  The preferred cross-linkers are polyallyl sucrose and polyallyl pentaerythritol, preferably present from 0.75 to 1.5% by weight with respect to the polymer.  The cross-linked polymer may be subjected to the action of steam to form a porous cake, the cake being dried and milled to a predetermined size.  The antacid and polymer are preferably present in the ratio by weight of 1 part of polymer to 1-6 parts of antacid.  For tabletting binders such as gelatin, gum acacia, tragacanth, pectin and diluents such as starch, talc and terra alba may be used.ALSO:An antacid preparation comprises a non-toxic antacid substance p  which is an oxide, hydroxide, carbonate or hydroxy salt of magnesium together with a non-toxic water-soluble synthetic polymer which in aqueous solution forms a gel at a pH above 4.5 but which remains fluid at a lower pH.  The polymer used should preferably buffer the antacid to lower the pH and increase in viscosity after passing into the duodenum to provide a bulking action.  Specified is a polymer of acrylic acid cross-linked with a polyhydroxy compound having at least 3 hydroxy groups, such as a polyhydroxy alcohol, wherein the hydrogen atoms of at least three hydroxyl groups are replaced with unsaturated aliphatic radicals preferably having 2-4 carbon atoms, or cross-linked with an unsaturated ether or unsaturated saccharide ether or unsaturated ester having at least three double bonds available for cross-linking, such as the tri-acrylic acid ester of glycerol and acrylic acid esters of glycerol having from 3-8 acid residues. The polyhydroxy compounds specified are glucose, fructose, mannose, galactose, sucrose, lactose, maltose, glycerol, erythritol, dulcitol, mannitol, sorbitol pentaerythritol.  The preferred cross-linkers are polyallyl sucrose and polyallyl pentaerythritol, preferably present from 0.75% to 1.5% by weight with respect to the polymer.  The cross-linked polymer may be subjected to the action of steam to form a porous cake, the cake being dried and milled to a predetermined size.  The antacid and polymer are preferably present in the ratio by weight of 1 part of polymer to 1-6 parts of antacid.  For tabletting binders such as gelatin, gum acacia, tragacanth, pectin and diluents such as starch, talc and terra alba may be used.