digoxin and quinine

Quinine inhibits P-glycoprotein (P-gp), a transporter responsible for the renal and biliary excretion of digoxin. This inhibition reduces digoxin clearance, leading to increased plasma digoxin concentrations. Additionally, both drugs can prolong cardiac repolarisation and have additive effects on the myocardium.

Elevated digoxin levels can precipitate toxicity, manifesting as nausea, vomiting, visual disturbances, bradyarrhythmias, and potentially life-threatening ventricular arrhythmias. The risk of digoxin toxicity is substantially increased, especially in patients with renal impairment, older adults, or those with pre-existing cardiac disease.

Avoid concurrent use if possible. If co-prescription is necessary, monitor serum digoxin concentrations, renal function, and electrocardiogram closely. Reduce the digoxin dose empirically (e.g., by 25% to 50%) and adjust based on digoxin levels and clinical response. Monitor for signs of digoxin toxicity.

The interaction is well documented in pharmacokinetic studies and case reports. Variations in the degree of digoxin level elevation have been observed, but clinically significant toxicity is widely reported when the combination is used without dose adjustment.