Drug interactions between Bactrim and potassium phosphate

MONITOR: The use of trimethoprim in combination with other potassium-sparing drugs or potassium salts may increase the risk of hyperkalemia. Trimethoprim inhibits sodium reabsorption and potassium excretion by blocking sodium channels in the renal distal tubules. Studies of patients treated with standard and high dosages of trimethoprim-sulfamethoxazole compared to similar controls treated with other antibiotics indicate that reversible increases in serum potassium are fairly common with trimethoprim use. Although generally asymptomatic, severe hyperkalemia including metabolic acidosis, paralysis, nonoliguric renal failure, and ventricular arrhythmia have been reported. Risk factors for developing hyperkalemia include use of high dosages of trimethoprim (e.g., for the treatment of MRSA skin infections or Pneumocystis jiroveci pneumonia (PCP) in AIDS patients); renal impairment or age-related decline in renal function; aldosterone or adrenal insufficiency; concomitant use of drugs that increase the risk of hyperkalemia (e.g., ACE inhibitors, angiotensin II receptor blockers, aldosterone antagonists; potassium-sparing diuretics); diets with potassium-rich foods (e.g., tomatoes, raisins, figs, baked potatoes, bananas, papayas, pears, cantaloupe, mangoes); and use of potassium salt substitutes.

MANAGEMENT: Serum potassium and sodium levels as well as renal function should be monitored during coadministration of trimethoprim with other potassium-sparing drugs or potassium salts, particularly in the elderly, in patients with renal impairment, and during high-dose or long-term trimethoprim treatment. A dosage reduction of trimethoprim is recommended in renal dysfunction (50% reduction for CrCl between 15 and 30 mL/min). Patients should be given dietary counseling to avoid excessive intake of potassium-rich foods and salt substitutes, and advised to seek medical attention if they experience symptoms of hyperkalemia such as weakness, irregular heartbeat, confusion, tingling of the extremities, or feelings of heaviness in the legs. Trimethoprim should be discontinued if hyperkalemia occurs.