Why does Magnesium Citrate Oral Solution need to be disposed of within 24 hours of opening bottle?

It is suggested you do this to avoid microbial contamination as it attracts moisture from the air. Magnesium Citrate does not decompose quickly. Under the proper conditions it should still be fine to continue using the oral solution after 24 hours. Consider refrigeration, tightly sealing the bottle, or removing the moisture in the bottle with silicon absorbents or a drying oven.