digoxin

Digoxin is one of cardiology's oldest medicines, descended from the foxglove plant. It does two useful things at once: it makes each heartbeat a little stronger, and it slows a fast heart rate, which is why it has long been used in heart failure and atrial fibrillation.

Its strengthening effect comes from blocking a cell pump (the sodium-potassium pump), which indirectly raises calcium inside heart muscle so each contraction is more forceful. Its rate-slowing effect comes mainly from boosting vagal nerve tone, which damps conduction through the AV node.

Today it is a second-line, add-on choice rather than a first move: it can ease symptoms and help control rate in atrial fibrillation, but unlike some heart-failure drugs it does not clearly lengthen life, so newer agents are preferred first.

Its defining caveat is a narrow safety margin. The helpful dose and the toxic dose sit close together, and low potassium or poor kidney function tips people toward toxicity — nausea, visual color changes, and dangerous rhythm disturbances — so blood levels and electrolytes are watched.