The answer, step by step

1. 1Trimethoprim blocks a sodium channelIn the kidney’s collecting ductThe same channel amiloride blocksWhy? →
2. 2Less sodium in, less potassium outThe kidney swaps sodium in for potassium outBlock the sodium, and potassium staysWhy? →
3. 3Potassium builds upElectrolyte imbalance is a common side-effectIt adds to other drugs that raise potassiumWhy? →
4. 4The heart’s rhythm is at riskWeakness, palpitationsSevere hyperkalaemia can cause dangerous arrhythmiasWhy? →

What to do

• Do thisCheck potassium in those at riskMonitor serum electrolytes if hyperkalaemia is likelyConsider it for others, especially on long-term use
• Do thisSpot the combinationACE inhibitor, ARB, spironolactone or NSAID?Each adds to the potassium rise
• UrgentPalpitations, weakness or collapseCould be dangerous hyperkalaemiaNeeds urgent medical assessment