Antacids

Antacids are inorganic salts — hydroxides, carbonates, or bicarbonates — that react directly with hydrochloric acid in the gastric lumen, forming water, a salt, and often carbon dioxide. This simple contact reaction gives near-immediate onset. But there is no receptor blocked and no enzyme inhibited, so once the antacid is buffered or the stomach empties, acid secretion resumes at baseline. This fast-on, fast-off profile suits episodic relief or pairing with a slower agent like a PPI, not monotherapy for erosive disease.

The Four Cation Families

Magnesium hydroxide neutralizes acid well but osmotically draws water into the bowel, causing diarrhea. In renal impairment, magnesium that would normally be excreted instead accumulates, risking hypermagnesemia with weakness, hypotension, and cardiac conduction disturbances.

Aluminum hydroxide has the opposite bowel effect — constipation, from slowed motility and an astringent gut action. Aluminum also binds intestinal phosphate into an insoluble complex excreted in stool, which can contribute to hypophosphatemia generally, but is deliberately exploited as a phosphate-binder strategy in chronic kidney disease.

Calcium carbonate neutralizes acid and doubles as a calcium supplement, but calcium stimulates gastrin release, causing rebound acid hypersecretion once buffering wears off. Chronic high-dose use risks milk-alkali syndrome: hypercalcemia, metabolic alkalosis, and renal impairment together.

Sodium bicarbonate neutralizes acid almost instantly and is highly soluble, but delivers a substantial sodium load and, since bicarbonate is absorbed, can cause systemic alkalosis — a poor choice with hypertension, heart failure, or sodium restriction.

Combination Products

Many antacids combine magnesium and aluminum hydroxide: magnesium's laxative tendency offsets aluminum's constipating tendency, yielding a more bowel-neutral product than either cation alone. Calcium carbonate is sometimes layered in too, adding its own rebound-acid consideration.

Chelation and Absorption Interference

The most clinically significant antacid issue often has nothing to do with stomach pH: their cations chelate — form insoluble complexes with — other drugs in the gut, blocking absorption. Examples include tetracycline and fluoroquinolone antibiotics, oral iron, levothyroxine, and some antiretrovirals; taking these with an antacid can render the other drug useless even though both were swallowed correctly. The fix is separating administration times so the two are not in the stomach together; the spacing needed varies by drug pair and should be confirmed against current labeling or a pharmacist.

Timing and Patient Teaching

Because antacids act briefly, timing relative to meals matters: taken with or after food, slower gastric emptying keeps the drug in the stomach longer, extending the buffering window versus an empty stomach. Patients should also learn that needing antacids with increasing frequency signals a need for evaluation, not more self-treatment — persistent symptoms may indicate an ulcer or reflux disease needing diagnosis and definitive therapy.