Anhydrous magnesium sulfate is used as a drying agent. Since the anhydrous form is hygroscopic (readily absorbs water from the air) and therefore harder to weigh accurately, the hydrate is often preferred when preparing solutions, for example in medical preparations.

Water is extremely difficult to remove from chemical compounds because solvents usually hold onto it well. ... Drying agents like magnesium sulfate (MgSO4) work by complexing with H2O in the solvent and forming a hydrated MgSO4 precipitate. This precipitate can then be gravity filtered out, yielding an anhydrous product.