As a pharmacist, you should be be careful when use of trimethoprim/sulfamethoxazole (Bactrim, Septrin, etc) in combination with drugs that increase potassium like angiotensin converting enzyme inhibitors (ACEI), angiotensin receptor blockers (ARBs) or spironolactone. Remmember that trimethoprim/sulfamethoxazole labeling has a precaution about hyperkalemia, and it happens more often than previously recognized. Trimethoprim can decrease urinary potassium excretion because it acts like the potassium-sparing diuretic spironolactone or amiloride.
Potassium can go up in anyone. But hyperkalemia is mainly a problem with high doses, renal insufficiency or when trimethoprim/sulfamethoxazole (TMP/SMX) is used with other drugs that can increase potassium. New evidence suggests that patients over age 65 are about 7 times more likely to be hospitalized for hyperkalemia if they take trimethoprim/sulfamethoxazole (TMP/SMX) with an ACE inhibitor or ARB. Watch for this interaction.
Hyperkalemia often occurs within 5 days of starting TMP/SMX. Suggest checking potassium on day 4 in patients who will take TMP/SMX longer than 5 days and who are at higher risk for hyperkalemia. These are patients taking TMP/SMX in combination with ACEIs, ARBs, aldosterone antagonists, potassium-sparing diuretics or potassium supplements. Also caution about patients with impaired renal function or those taking high doses for MRSA or Pneumocystis pneumonia. Or suggest another antibiotic such as doxycycline or clindamycin for community-acquired methicillin-resistant Staph aureus. Also use a lower dose for renal insufficiency (CrCl < 30 mL/min). Advise patients on medications that increase potassium to be careful about salt substitutes, they can contain a lot of potassium.
REFERENCES
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Antoniou, T. (2010). Trimethoprim-Sulfamethoxazole–Induced Hyperkalemia in Patients Receiving Inhibitors of the Renin-Angiotensin System. Archives of Internal Medicine, [online] 170(12), p.1045. Available at: https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/416098
Alappan, R. (1996). Hyperkalemia in Hospitalized Patients Treated with Trimethoprim-Sulfamethoxazole. Annals of Internal Medicine, 124(3), p.316. Available at: https://www.acpjournals.org/doi/10.7326/0003-4819-124-3-199602010-00006