Adverse effect of orally administered magnesium hydroxide on serum magnesium concentration and systemic acid-base balance in adult cattle

Summary Two cows, 1 with diarrhea and 1 with signs of forestomach outflow obstruction, were treated in part with repeated doses of a commercially available antacid/cathartic preparation containing magnesium hydroxide. Both cows subsequently were determined to have hypermagnesemia, along with severe metabolic alkalosis. In addition, each cow was comatose at the time of death. A clinical study was initiated to investigate the causal relationship between the ingestion of magnesium hydroxide and the generation of hypermagnesemia and metabolic alkalosis in adult cows. Twelve healthy mature cross bred beef cows were allotted at random to a fed or fasted (simulated anorexia) group, with 2 untreated and 4 treated cows in each group. A single dose of magnesium hydroxide (1.5 g/kg of body weight, suspended in 3.8 L of warm water) was administered per os to each treated cow, whereas each control cow was given only water. Individual determinations of selected venous blood gas values (plasma bicarbonate ion [HCO3‒] concentration and base excess [be]), serum magnesium (Mg) concentration, and urine magnesium fractional clearance ratio (Mg-fcr) were made immediately before drug administration, and at 2, 4, 8, 12, 24, and 36 hours after treatment. Compared with control values at the conclusion of the study, mean serum Mg concentration, urine Mg-fcr, plasma HCO3‒ concentration, and be were significantly higher (P = 0.0001, P = 0.0015, P = 0.028, and P = 0.021, respectively) in treated cows. Throughout the study, serum Mg concentration and urine Mg-fcr in the cows increased linearly, independent of diet, whereas the increase in mean plasma HCO3‒ concentration and be over time did not develop a well-defined response pattern because of a substantial diet-treatment-time interaction during the first 12 hours after treatment. It was concluded that oral administration of magnesium hydroxide has an adverse effect on serum Mg concentration and acid-base balance in healthy fasted or fed cows. Therefore, veterinarians should be aware of potential metabolic alkalosis and considerable increase in the magnesium load in the blood of cattle as side effects of higher than recommended orally administered doses of magnesium hydroxide.