We report the case of a patient born in 1923 who had been treated with metformin since 1977 and experienced panic attacks. Lactic acid levels, measured during an attack for the first time since metformin was prescribed, were found to be elevated on two occasions. Metformin was discontinued, and during a 9-month follow-up period the patient was free from any psychiatric disorder. The role of lactic acid in the triggering of panic attacks is discussed.