Pharmacokinetic Studies of Orally Administered Magnesium Oxide in Rats

Magnesium oxide (MgO) tablets are widely used as laxatives in patients with constipation. Recently, the "Revision of Precautions on the Use of Magnesium Oxide" has been issued by the Japanese Pharmaceuticals and Medical Devices Agency, warning against the risk of hypermagnesemia with the use of MgO. However, the majority of physicians continue to administer MgO for constipation without adequately considering its safe use. In the present study, we performed two analyses using an identical lot of MgO tablets and evaluated the risk of hypermagnesemia. Approximately 90% of the MgO tablets dissolved within 120 min in dissolution testing; it was believed to form an absorbable state for magnesium. With orally administered MgO, 15% is absorbed in the body and 85% is excreted via the feces without being detected in pharmacokinetic analysis. Magnesium absorbed into the plasma demonstrated peak concentration 3 h after administration and was excreted via the urine within 48 h.