Editor—Welfare et al advocate aggressive potassium replacement in gastroenteritis associated with profound hypokalaemia.1 In such cases it is vital to measure serum magnesium concentrations too, for three reasons. Firstly, hypomagnesaemia occurs fairly commonly in states of high intestinal output. Secondly, it often coexists with severe hypokalaemia and may exacerbate clinical effects, particularly cardiac arrhythmias.2 Thirdly, hypomagnesaemia from any cause can lead to potassium wasting and thus render coexisting hypokalaemia resistant to replacement treatment if magnesium is not replenished simultaneously.3
