Pharmacology Notes

Antacids
There are differences in the types of antacids, in terms of their contents, neutralizing capacity, duration of action, side eff ects, and cost. These must be considered when choosing an antacid for therapeutic use. Antacids are OTC drugs. The most widely used antacids are sodium bicarbonate, calcium carbonate, aluminum hydroxide, and magnesium hydroxide.

Mechanism of Action
Antacids neutralize hydrochloric acid and raise gastric pH, thus inhibiting pepsin (a gastric enzyme). Antacids reduce the concentration and total load of acid in the gastric contents. By increasing gastric pH, antacids also inhibit pepsin activity. In addition, they strengthen the gastric mucosal barrier.

Indications
These agents are used widely for the relief of heartburn, dyspepsia, and medical treatment of peptic ulcer. Th e primary role of antacids in the management of acid-peptic disorders is the relief of pain. Nonsystemic antacids (magnesium or aluminum substances) are preferred to systemic antacids such as sodium bicarbonate for intensive ulcer therapy because they avoid the risk of alkalosis. Liquid antacid forms have a greater buff ering capacity than tablets. However, tablets are more convenient to carry. Antacid mixtures such as aluminum hydroxide with magnesium hydroxide provide more even, sustained action than single-agent antacids, and permit a lower dosage of each compound.

Adverse Effects
Constipation can occur in patients using calcium carbonate and aluminum containing antacids. Diarrhea is a common adverse effect of magnesium- and sodium-containing antacids. If diarrhea occurs, the patient may alternate the antacid mixture with aluminum hydroxide. Hypophosphatemia and osteomalacia can occur with long-term use of aluminum hydroxide, but patients, such as alcoholics. Calcium carbonate usually is avoided because it causes acid rebound, hypercalcemia, vomiting, metabolic alkalosis, confusion, and renal calculi. It may also delay pain relief and ulcer healing.

Contraindications and Precautions
Antacids are contraindicated in patients with severe abdominal pain of unknown cause, and during lactation. Sodium bicarbonate is contraindicated in patients with hypertension, congestive heart failure (CHF), severe renal disease, and edema. It should not be used for ulcer therapy. All antacids should be used cautiously in elderly patients and renally impaired patients. Chronic administration of calcium carbonate-containing antacids should be avoided because of hypercalcemia. Calcium carbonate and magnesiumcontaining antacids should be used cautiously in patients with severe renal disease.

Drug Interactions
Because antacids alter gastric pH and affect absorption of ingested substances, they have a high potential for drug interactions. To ensure consistent absorption and therapeutic effi cacy, orally administered drugs should be given 30 to 60 minutes before antacids. Th ese agents bind with tetracycline and inhibit its absorption, reducing its therapeutic efficacy.
Antacids may destroy the coating of enteric-coated drugs, leading to premature drug dissolution in the stomach. Antacids may interfere with the absorption of many drugs, including cimetidine, ranitidine, digoxin, isoniazid, iron products, anticholinergics, and phenothiazines