The elderly patients are susceptible to acute renal failure due to dehydration or infection. Therefore, the drug should be administered with caution. We report two cases of acute renal failure from dehydration that led to a subsequent drug poisoning. Case 1: An 85-year-old woman with a history of colorectal cancer surgery was admitted to our emergency department for appetite loss and weakness. Because she was given a normal amount of drugs under the condition of poor oral intake, she was hospitalized by digitalism. Case 2: A 72-year-old woman was admitted to our emergency department for disturbance of consciousness and appetite loss. The medication given by a staff in geriatric health services facility appeared to have caused a pilsicainide poisoning. As the elderly patients were given a normal amount of drugs under the poor oral intake condition, blood levels of renal excretion type drug had increased in both cases. Medication management for the elderly should be comprehensively considered the background of the individual.