Mineral-metabolic side effects of a low dose of a conventional Al-Mg antacid were examined in 10 healthy volunteers, who were given one antacid tablet after the 3 main meals and at bedtime (buffering capacity, 120 mmol/day) for 4 weeks. Compared with the pre-medication state, the following changes of statistical significance occurred during antacid ingestion: increase in urinary excretion of magnesium, calcium, and aluminum; decrease in urinary excretion of phosphate; increase in maximal renal phosphate reabsorption (Tm PO4/GFR); and increase in serum concentration of aluminum. Most of the changes were normalized 3-4 days after cessation of antacid medication. There was no change in intestinal absorption of calcium. The fact that even this low dose of antacids can provoke measurable changes in mineral metabolism, including aluminum absorption, is noteworthy, although we did not see any clinical symptoms from the biochemical changes.