Loop diuretics, like furosemide, cause three main toxicities: diuresis-related fluid and electrolyte abnormalities, ototoxicity, and hypersensitivity reactions. A potassium of 2.9 mmol/L is quite low—once levels dip below 2.5 mmol/L, lethal cardiac arrhythmias and severe muscle weakness may occur. The furosemide should be held until the electrolytes can be normalized. Salt substitute is an inefficient way to replete potassium—IV potassium is likely required for severe deficiency. An oral medication can be used for less profound hypokalemia. While hydration status should be assessed, the medication should be withheld.