Basic. That's why they call them antacids.

In some cases neither. Some are designed to inhibit the production of stomach acid instead of neutralising some of what is there.

They are generally basic, but they could also be amphoteric: 1) "An antacid is any substance, generally a base, which counteracts stomach acidity. In other words, antacids are stomach acid neutralizers." "Antacids perform a neutralization reaction, i.e. they buffer gastric acid, raising the pH to reduce acidity in the stomach." "Side effects - Aluminum hydroxide: may lead to the formation of insoluble aluminium-phosphate-complexes, with a risk for hypophosphatemia and osteomalacia. Although aluminium has a low gastrointestinal absorption, accumulation may occur in the presence of renal insufficiency. Aluminium-containing drugs may cause constipation. - Magnesium hydroxide: has laxative properties. Magnesium may accumulate in patients with renal failure leading to hypermagnesemia, with cardiovascular and neurological complications. See Milk of magnesia. - Carbonate: regular high doses may cause alkalosis, which in turn may result in altered excretion of other drugs, and kidney stones. A chemical reaction between the carbonate and hydrochloric acid may produce carbon dioxide gas. This causes gastric distension which may not be well tolerated. - Calcium: compounds containing calcium may increase calcium output in the urine, which might be associated to renal stones. Calcium salts may cause Constipation. - Sodium: increased intake of sodium may be deleterious for arterial hypertension, heart failure and many renal diseases." Source and further information: http://en.wikipedia.org/wiki/Antacid 2) "When food is eaten gastric juice is secreted into the stomach by parietal glands. The gastric juice contains hydrochloric acid and a precursor to the enzyme pepsin. The gastric juice is highly acidic with a pH of about 0.8. As the gastric juice is mixed with mucus and food the pH rises to about 2.0. The acidic gastric juice, along with the enzyme pepsin, starts to break down the proteins contained in the foodinto amino acids. If the sphincter between your esophagus and stomach fails to close during digestion, gastric juice can enter the esophagus and cause a burning sensation. This is known as heartburn. The gastric juice can also burn the lining of the stomach. The parietal glands can also excrete excess gastric juice when irritated by overeating or drinking certain foods including tomato sauces, citrus products, caffeine and alcohol. In addition, stress and medical disorders can sometimes cause excess production of gastric juice. If heartburn remains untreated for extended periods, stomach or duodenal ulcers may develop. Antacids are commonly used to treat heartburn. The active ingredient present in any antacid is a base. This base reacts with and neutralizes the excess acid. Different brands of antacids in the supermarkets contain different active ingredients. TumsTM contains calcium carbonate (CaCO3). Alka-Selzer TM contains sodium bicarbonate (NaHCO3). RolaidsTM contain aluminum sodium dihydroxy carbonate (AlNa(OH)2CO3) or calcium carbonate (CaCO3). Mylanta and Maalox contain magnesium hydroxide (Mg(OH)2). These antacids will neutralize the hydrochloric acid according to the following reactions: NaHCO3 + HCl -> NaCl + CO2 + H2O CaCO3 + 2 HCl -> CaCl2 + CO2 + H2O AlNa(OH)2CO3 + 4 HCl -> CaCl2 + CO2 + H2O Mg(OH)2 + 2 HCl -> MgCl2 + 2 H2O " Source and further information: http://www.nvcc.edu/alexandria/science/EfficiencyofAntacidTablets.html 3) "Aluminium hydroxide is amphoteric. It dissolves in acid, forming Al(H2O)63+ or its hydrolysis products. It also dissolves in strong alkali, forming Al(OH)4-." Source and further information: http://en.wikipedia.org/wiki/Aluminum_hydroxide 4) "In chemistry, an amphoteric substance is one that can react as either an acid or base. The word is derived from the Greek prefix ampho- (αμφί-) which means both and the suffix -ic (-ικÏŒς) which means the attribute that the given substance has to react either as acid or as base." Source and further information: http://en.wikipedia.org/wiki/Amphoterism 5) "Magnesium hydroxide is not very soluble in water, with a Ksp of 1.5x10-11. While the solubility of magnesium hydroxide is low, all of the magnesium hydroxide that does dissolve in the water does dissociate. Since the dissociation of this small amount of dissolved magnesium hydroxide is complete, magnesium hydroxide is considered a strong base." Source and further information: http://en.wikipedia.org/wiki/Magnesium_hydroxide 6) "In aqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together in a dynamic equilibrium. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate ion is prevalent. In more acid conditions, aqueous carbon dioxide, CO2(aq), is the main form, which, with water, H2O, is in equilibrium with carbonic acid - the equilibrium lies strongly towards carbon dioxide. Thus sodium carbonate is basic, sodium bicarbonate is weakly basic, while carbon dioxide itself is a weak acid." Source and further information: http://en.wikipedia.org/wiki/Carbonate