Magnesium plays an important role in the body as a cofactor for DNA and protein synthesis, oxidative phosphorylation, neuromuscular excitability, enzyme activity and regulation of parathyroid hormone (PTH) secretion.1 The regulation of plasma magnesium concentration is balanced by intestinal absorption and renal excretion. Hypomagnesaemia (variously defined but taken here as plasma magnesium<0.7 mmol/L) may result from inadequate magnesium intake, increased gastrointestinal or renal loss or redistribution from extracellular to intracellular space. In addition, hypomagnesaemia has been associated with the use of a number of drugs and, in 2012, the Medicines and Healthcare products Regulatory Agency (MHRA) drew attention to the risk associated with prolonged use of proton pump inhibitors (PPIs).2 Here we review the causes and management of hypomagnesaemia.