Mr J with leg cramps and low potassium (2.9 mmol/L) who takes ramipril, salbutamol, furosemide, amlodipine and digoxin. Which drug is most likely to have caused the effect?

• A. Ramipril
• B. Furosemide
• C. Digoxin
• D. Amlodipine

Answer: (B)

The main idea is that low potassium with leg cramps points to a potassium-wasting effect from a diuretic. Furosemide is a loop diuretic that blocks the Na-K-2Cl transporter in the thick ascending limb of the loop of Henle, causing increased excretion of sodium, water, and potassium in the urine. This can lead to significant hypokalemia, which fits the potassium of 2.9 mmol/L and the cramping symptoms.

Other drugs in the list don’t fit as well for causing a marked drop in potassium: ramipril tends to raise potassium by reducing aldosterone-driven potassium loss; amlodipine has little impact on potassium; digoxin toxicity is a risk when potassium is low, but the drug that caused the low potassium is the loop diuretic; salbutamol can cause a modest shift of potassium into cells, but it’s less likely to produce such a pronounced hypokalemia compared with a loop diuretic.