Antacids and Their Properties

Antacids

• Antacids neutralize excess stomach acid
• Antacids do not inhibit gastric acid secretion

Antacid Compounds

• Common antacids are weak inorganic bases
• Examples include calcium carbonate, sodium bicarbonate, magnesium trisilicate, magnesium hydroxide (milk of magnesia), and aluminum hydroxide

Acid Neutralizing Capacity (ANC)

• ANC is a measure of an antacid’s ability to neutralize acid
• FDA requires antacids to have greater than 25% ANC
• Minimum dose of drug added to 10 mL of 0.5 N HCl and 40 mL of water must raise pH to 3-5 at end of 10 min at 300 rpm

Calcium Carbonate

• Potent and fast-acting antacid
• About one-third of the administered calcium is absorbed
• Avoid in patients with hypercalcemia
• Can raise the pH of the blood causing alkalosis
• Calcium may cause constipation \

Magnesium Hydroxide

• Insoluble powder that forms magnesium chloride in the stomach
• Potent antacid effect
• Mainly not absorbed
• Can induce diarrhea
• Encephalopathy in cases of renal impairment

Magnesium Trisilicate

• Compound of magnesium oxide and silica
• Insoluble powder that reacts slowly with the gastric juice
• Less potent than magnesium hydroxide
• Can induce diarrhea
• Silica-based renal stones reported on long-term treatment

Aluminum Hydroxide

• Forms aluminum chloride in the stomach
• Raises the pH of gastric juice up to 4
• Gradual action with effects lasting for several hours
• May be absorbed and affect CNS if renal function is impaired
• Can cause constipation
• Aluminum hydroxide combines with phosphates in the GIT

Sodium Bicarbonate

• Water soluble
• Rapidly raises pH up to 7.4
• Carbon dioxide liberation causes belching
• Large doses or frequent administration can cause alkalosis
• Sodium absorption can induce hypertension
• Caution in patients with hypertension, cardiac failure, or chronic kidney disease

Choice of Antacid

• Considerations include underlying conditions and patient preference

Pregnancy and Breastfeeding

• Generally safe for use
• Aluminum-containing antacids are safe in pregnancy

Common Antacid Preparations

• Calcium carbonate - Bisodol, Calcichew
• Magnesium salts - Milk of Magnesia, Maalox
• Aluminum salts - Maalox, Gaviscon, Actonel

General Side Effects

• Antacids are relatively harmless but can have minor contraindications
• May impair normal bowel function
• Some preparations contain salt posing clinical implications
• Reduce gastric acidity leading to problems with nutrient absorption

Alginates

• Glucuronic-mannuronic acid polymer.
• Forms hydrogels that act as a barrier.
• Reduces reflux in esophagus.
• Main ingredient in Gaviscon.

Alginates and Antifoaming Agents

• Often combined with alginates and antifoaming agents.
• Alginates form a neutralizing layer preventing reflux of stomach acids.
• Help prevent acid reflux or heartburn.
• Antifoaming agents such as simethicone (dimethicone) prevent gas formation and reduce flatulence.

Clinical Uses

• Treatment of GORD (infrequent heartburn).
• Treatment of duodenal ulcers, although anti-H2 and proton pomp inhibitors are more effective.
• No prevention of NSAID-associated ulcers.

Treatment of GORD

• Effective for patients with infrequent heartburn or isolated episodes.
• Antacids provide rapid acid neutralization and faster symptomatic relief.
• Their effect is short-lived (1-2 h).