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Mephedrone is a cathinone derivative that has been used as a ‘legal high’ but was re-classified in the UK as a class B (intermediate category) drug. This report describes an 18-year-old man with type 1 diabetes who developed ketoacidosis following self-reported mephedrone use. Cathinone compounds may directly increase the risk of diabetic ketoacidosis by stimulating the central nervous system but may also indirectly impair an individual's ability to manage their diabetes through changes in cognitive function and behaviour.