Aluminum-based antacids cause constipation, and magnesium-based products cause diarrhoea. When combination products are used, diarrhoea tends to predominate as a side effect. Although these are termed 'non-absorbable', a proportion of aluminum and magnesium is absorbed and the potential for toxicity exists, particularly with coexistent renal failure.
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Source: Clinical Pharmacy and Therapeutics, 4th ed. by Roger Walker and Cate , p. 164
