One of the causes of preoperative hypokalemia is the prolonged use of herbal medicines, especially licorice. Licorice can induce pseudo-aldosteronism, hypertension, metabolic alkalosis, and hypokalemia. An 87-year-old woman with a history of knee osteoarthritis was scheduled to undergo a total knee arthroplasty (TKA) under spinal anesthesia. She had also been prescribed herbal medicine for osteoarthritis of the knee two years before the surgery. During the surgery, the pulse oximeter showed hypoxemia. After the surgery was completed, arterial blood sampling showed hypoxemia, hypokalemia with electrocardiography (ECG) abnormalities, and metabolic alkalosis. The symptoms improved after the discontinuation of herbal medicines and administering potassium chloride. It is necessary to suspect electrolyte abnormalities as one of the causes of hypoxemia, hypertension, or ECG abnormalities in patients prescribed herbal medicines. Therefore, it is also important to ensure that patients on such drugs have their blood potassium levels assessed frequently in the perioperative period.