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A 71-year-old woman, who had been taking lansoprazole for 18 months for dyspepsia, presented with vomiting, thought to be due to gallstones, and was found to have severe hypomagnesaemia. She was treated with intravenous and then oral magnesium, and discharged, but was soon readmitted with symptoms due to hypomagnesaemia, and again treated with magnesium supplementation. No other recognised cause for hypomagnesaemia was found. Because of recent reports of hypomagnesaemia due to other proton pump inhibitors, lansoprazole was changed to ranitidine. Her symptoms resolved and the serum magnesium returned to normal. Oral magnesium supplementation was stopped with no return of symptoms or hypomagnesaemia. Such an association must be borne in mind with suggestive symptoms in patients on long term proton pump inhibitors; their cessation or change to H(2) receptor antagonists is likely to correct the situation rapidly.