Milk of magnesia is a saline type laxative.
This means that the magnesia acts to pull a salt or saline solution into the intestines from the blood. This excessive fluid then provides a flushing or laxative action within the intestines. This preparation is useful for the temporary relief of constipation. It can also be used for indigestion or heartburn, but the diarrhea side effect usually limits its use. Follow your physician's instructions carefully.