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We describe the case of a complex, frail, multimorbid patient in her 80s who developed severe hyponatraemia while being treated with oral rifampicin for prosthetic hip joint infection by our community-based hospital at home service.Given her complex medical history, there were several potential causes for her hyponatraemia which were considered but subsequently refuted as the likely explanation.It became apparent that the chronology of the onset of the hyponatraemia correlated well with the initiation of rifampicin. Once the rifampicin was withdrawn, the hyponatraemia gradually resolved.This case is an important reminder that there are many causes of hyponatraemia, which can include drugs which have not previously been reported as common precipitants.