Succinic acid cibenzoline (CZ) is an antiarrhythmic agent often used for the treatment of tachyarrhythmia. However, hypoglycemia should be avoided in the treatment of diabetes. We herein report two late-stage elderly subjects who experienced a severe and prolonged hypoglycemic coma after the usage of CZ. These cases suggest that, when CZ is administered to elderly subjects with renal dysfunction and/or frailty, we should be aware of the possibility that this medicine may induce hypoglycemia and should adjust the dose as appropriate and monitor the concentration of CZ to avoid severe hypoglycemia.