A 42-year-old woman comes to the office due to frequent episodes of burning in her chest and small amounts of regurgitation after meals and at nighttime. Medical history includes hypertension. Vital signs are within normal limits. BMI is 30 kg/m2. There is no abdominal tenderness and the remainder of the physical examination is normal. The patient shows the clinician an over-the-counter antacid that she has been taking to relieve her symptoms. The preparation contains a combination of magnesium and aluminum hydroxide. Which of the following is the most likely rationale for combining both mineral salts in this antacid preparation?
Magnesium salts (eg, magnesium trisilicate, magnesium hydroxide) and aluminum hydroxide are weak alkali mineral salts. They temporarily increase the gastric pH by neutralizing hydrochloric acid, helping to relieve gastroesophageal reflux symptoms.
Aluminum hydroxide has a tendency to cause constipation due to interactions with intestinal secretions that form insoluble salts. In contrast, magnesium salts cause osmotic diarrhea. Therefore, the two medications are combined to offset the adverse effects of the individual medications. Patients with reflux symptoms and chronic constipation may benefit from magnesium salt monotherapy, whereas aluminum hydroxide monotherapy may be of value in patients with chronic diarrhea.
(Choice A) Aluminum, when absorbed in large doses, causes osteomalacia, bone pain, hypercalcemia, and dementia. Aluminum absorption in the gastrointestinal tract is minimal from the salt form, and toxicity tends to occur in patients with chronic kidney disease that limits renal excretion. Combination therapy does not alter systemic absorption.
(Choice B) Drug interactions are common with antacid medications, largely due to pH-related changes in protein binding, absorption, or elimination of the drugs. Combining multiple alkalinizing agents would not minimize their effects on other drugs.
(Choice C) Rebound acid hypersecretion can occur with calcium carbonate, magnesium hydroxide, aluminum hydroxide, and proton pump inhibitors. This is likely due to increased gastrin release as a result of gastric alkalinization or (in the case of calcium salts) direct ionic stimulation.
(Choice E) Milk-alkali syndrome is caused by excessive use of calcium carbonate antacids and is characterized by hypercalcemia, alkalosis, and renal dysfunction. Kidney injury occurs due to both hypercalcemia-induced renal vasoconstriction and calcium-induced diuresis. The resultant volume depletion, decreased glomerular filtration rate, and alkali intake lead to a metabolic alkalosis. However, severe alkalosis can occur with all antacids, and the combination of magnesium and aluminum would not decrease this risk.
Educational objective:
Magnesium salts and aluminum hydroxide are basic mineral salts used to neutralize gastric acid and relieve gastroesophageal reflux symptoms. Side effects include diarrhea and constipation, respectively. Therefore, these medications are often combined to offset the adverse effects of the individual medications.