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Wednesday, August 20, 2025

Antacids


Introduction

Antacids are among the oldest and most widely used medications in gastrointestinal (GI) pharmacotherapy. They are alkaline substances that neutralize gastric hydrochloric acid (HCl), thereby reducing acidity in the stomach and providing symptomatic relief from conditions such as gastroesophageal reflux disease (GERD), peptic ulcer disease (PUD), dyspepsia, and gastritis.

While more advanced acid-suppressing drugs such as H2 receptor antagonists (H2RAs) and proton pump inhibitors (PPIs) have largely replaced antacids for long-term management of acid-related disorders, antacids remain important for rapid, short-term relief of symptoms.

Mechanism of Action

Antacids work via direct chemical neutralization of gastric acid:

  • Hydrochloric acid (HCl) in the stomach reacts with the basic compound in antacids to form water and a salt, raising gastric pH.

  • This increase in gastric pH provides:

    • Relief from epigastric pain and heartburn

    • Reduced activation of pepsin (inactive above pH 4)

    • Indirect mucosal protection

Antacids act within minutes of administration but their duration of action is short (30–60 minutes) when taken on an empty stomach and 2–3 hours if taken after meals.

Types of Antacids and Representative Agents

1. Aluminum-containing antacids

  • Generic names: Aluminum hydroxide, aluminum phosphate

  • Mechanism: Neutralize gastric acid and form aluminum chloride salts.

  • Dose:

    • Aluminum hydroxide: 500–1500 mg orally every 3–6 hours as needed.

  • Advantages: Long duration, low systemic absorption.

  • Adverse effects: Constipation, hypophosphatemia (due to phosphate binding).

2. Magnesium-containing antacids

  • Generic names: Magnesium hydroxide, magnesium carbonate, magnesium trisilicate

  • Mechanism: Neutralize gastric acid to form magnesium chloride and water.

  • Dose:

    • Magnesium hydroxide (“milk of magnesia”): 400–1200 mg orally every 4–6 hours.

  • Advantages: Rapid onset.

  • Adverse effects: Diarrhea, hypermagnesemia (especially in renal failure).

3. Calcium-containing antacids

  • Generic names: Calcium carbonate

  • Mechanism: Neutralizes acid to form calcium chloride and carbon dioxide.

  • Dose:

    • Calcium carbonate: 500–1000 mg orally as needed; maximum 7 g/day.

  • Advantages: Provides calcium supplementation, rapid relief.

  • Adverse effects: Hypercalcemia, metabolic alkalosis (“milk-alkali syndrome”), constipation, rebound acid hypersecretion.

4. Sodium bicarbonate

  • Generic name: Sodium bicarbonate

  • Mechanism: Reacts rapidly with HCl, forming sodium chloride, water, and carbon dioxide.

  • Dose: 325–2000 mg orally every 4 hours as needed (max 7 g/day).

  • Advantages: Rapid onset, effective short-term.

  • Adverse effects: Systemic alkalosis, sodium overload (risk in hypertension, heart failure, renal disease), belching and gastric distention due to CO₂.

5. Combination antacids

Most commercial preparations combine aluminum and magnesium salts to balance side effects:

  • Examples:

    • Aluminum hydroxide + Magnesium hydroxide (Maalox®, Mylanta®)

    • Calcium carbonate + Magnesium hydroxide (Rolaids®)

    • Simethicone is often added to reduce bloating.

Clinical Uses

Antacids are indicated for:

  • Symptomatic relief of heartburn and indigestion (most common use).

  • Gastroesophageal reflux disease (GERD): Provide rapid relief but not long-term control.

  • Peptic ulcer disease (PUD): Previously used as primary therapy before PPIs/H2RAs; now reserved for symptomatic relief.

  • Gastritis: Reduce acidity and provide comfort.

  • Stress-related mucosal damage (adjunctive).

Dosage and Administration

  • Best taken after meals and at bedtime for prolonged effect.

  • Liquid suspensions are more effective than tablets due to faster neutralization.

  • Frequent dosing is required due to short duration of action.

Adverse Effects

Local GI Effects

  • Aluminum salts: Constipation.

  • Magnesium salts: Diarrhea.

  • Calcium carbonate: Constipation, flatulence.

  • Sodium bicarbonate: Belching, gastric distention.

Systemic Effects

  • Electrolyte imbalances: Hypercalcemia, hypermagnesemia, hypophosphatemia.

  • Metabolic alkalosis: Especially with sodium bicarbonate.

  • Milk-alkali syndrome: High calcium intake + absorbable alkali → hypercalcemia, alkalosis, renal failure.

Drug-induced complications in vulnerable groups

  • Renal impairment → accumulation of aluminum or magnesium → toxicity.

  • Heart failure or hypertension → sodium overload from sodium bicarbonate.

Contraindications

  • Severe renal impairment: Risk of magnesium or aluminum toxicity.

  • Hypercalcemia or nephrolithiasis: Contraindicated with calcium carbonate.

  • Congestive heart failure, hypertension, cirrhosis: Avoid sodium bicarbonate due to sodium load.

  • Metabolic alkalosis: Sodium bicarbonate and calcium carbonate may worsen alkalosis.

Precautions

  • Limit chronic use; symptomatic treatment only.

  • Monitor serum electrolytes with prolonged use.

  • Consider phosphate supplementation in chronic aluminum hydroxide users.

  • Use with caution in pregnancy (calcium carbonate preferred; sodium bicarbonate avoided due to fluid overload risk).

Drug Interactions

Antacids alter the absorption of many drugs through changes in gastric pH or binding interactions:

  • Reduced absorption (due to increased pH):

    • Ketoconazole, itraconazole (require acidic environment).

    • Iron supplements.

    • Tetracyclines, fluoroquinolones (chelation with calcium/magnesium/aluminum).

    • Levothyroxine.

    • Bisphosphonates.

  • Increased absorption/toxicity:

    • Aluminum hydroxide increases digoxin absorption.

  • Altered urinary excretion (alkalinization):

    • May decrease excretion of weak bases (e.g., amphetamines).

    • May increase excretion of weak acids (e.g., aspirin, methotrexate).

Recommendation: Separate antacid dosing from interacting drugs by at least 2–4 hours.

Clinical Comparisons

  • Rapid onset, short action: Sodium bicarbonate, calcium carbonate.

  • Slower onset, longer action: Aluminum and magnesium hydroxide.

  • Balanced formulations: Aluminum + magnesium combinations minimize constipation/diarrhea.

  • Best for long-term supplementation: Calcium carbonate (for calcium-deficient patients, but monitor for milk-alkali syndrome).

Role in Therapy Today

  • Antacids remain first-line OTC agents for immediate relief of heartburn and dyspepsia.

  • For chronic acid-related disorders (GERD, peptic ulcers), PPIs and H2RAs are superior for healing and prevention of recurrence.

  • However, antacids are valuable for on-demand therapy, in patients intolerant to other agents, or as adjuncts to more potent acid-suppressive medications.

Future Perspectives

  • Development of antacids combined with alginates (e.g., Gaviscon®) that form a floating gel barrier on gastric contents to reduce reflux.

  • Efforts to improve palatability, reduce rebound acid secretion, and enhance patient compliance.

  • Use in combination therapy with simethicone to address bloating and flatulence.

Summary of Key Agents and Doses

  • Aluminum hydroxide: 500–1500 mg PO q3–6h (constipation risk).

  • Magnesium hydroxide: 400–1200 mg PO q4–6h (diarrhea risk).

  • Calcium carbonate: 500–1000 mg PO as needed (risk of milk-alkali syndrome).

  • Sodium bicarbonate: 325–2000 mg PO q4h as needed (risk of alkalosis, sodium overload).

  • Combinations (Maalox®, Mylanta®, Rolaids®): Aluminum + magnesium ± simethicone.




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