Q) At a lecture I recently attended, the speaker said sulfamethoxazole/trimethoprim is a potentially dangerous medication. I use it all the time. Is there any data to support her comments? Where did she get her information?
Sulfamethoxazole/trimethoprim (SMX/TMP) is a combination of two antibiotics, each of which has the potential to interact with other substances.
It is well documented that sulfamethoxazole can inhibit the metabolism of cytochrome P450 2C9 substrates. Frequently prescribed medications that also use the cytochrome substrate include warfarin and oral antihypoglycemic agents.
Trimethoprim’s distinct properties also lead to drug interactions. Trimethoprim inhibits sodium uptake by the appropriate channels in the distal tubule of the kidney, preventing reabsorption and altering the electrical balance of the tubular cells. As a result, the amount of potassium excreted into the urine is reduced, yielding an accumulation of serum potassium.1
High serum potassium retention can manifest as hyperkalemia in patients with chronic kidney disease (CKD). Use of potassium-sparing drugs by patients with comorbidities, including CKD, can increase risk for hyperkalemia; concurrent use of these drugs with ACE inhibitors or angiotensin II receptor blockers (ARBs) compounds the risk.2 The first reports of hyperkalemia with trimethoprim use occurred in HIV patients treated with large doses for Pneumocystis carinii infection.3
In a population-based case-control study, the results of which were published in the British Medical Journal, Fralick and colleagues analyzed data on older patients (age 66 or older) who were taking either ACE inhibitors or ARBs in combination with an antibiotic.4 They found a significantly increased risk for sudden death within seven days of prescription of SMX/TMP, compared to amoxicillin; a secondary analysis also revealed an increased risk for sudden death within 14 days with SMX/TMP. The researchers speculated that this excess risk, which translated to 3 sudden deaths in 1,000 patients taking SMX/TMP versus 1 sudden death in 1,000 patients taking amoxicillin, “reflects unrecognized arrhythmic death due to hyperkalemia.”
Since more than 250 million prescriptions for ACE inhibitors/ARBs and 20 million prescriptions for SMX/TMP are written each year, there will be instances of overlap. The prudent clinician would prescribe a different antibiotic or, if avoidance is not possible, use the lowest effective dose and duration of SMX/TMP. Close monitoring of serum potassium levels is warranted in patients with comorbidities, especially CKD, who are taking ACE inhibitors or ARBs—and of course, in our geriatric population. —DLC
Debra L. Coplon, DNP, DCC
City of Memphis Wellness Clinic, Tennessee
1. Velazquez H, Perazella MA, Wright FS, Ellison DH. Renal mechanism of trimethoprim-induced hyperkalemia. Ann Intern Med. 1993;119:296-301.
2. Horn JR, Hansten PD. Trimethoprim and potassium-sparing drugs: a risk for hyperkalemia. www.pharmacytimes.com/publications/issue/2011/February2011/DrugInteractions-0211. Accessed August 24, 2015.
3. Medina I, Mills J, Leoung G, et al. Oral therapy for Pneumocystis carinii pneumonia in the acquired immunodeficiency syndrome: a controlled trial of trimethoprim-sulfamethoxazole versus trimethoprim-dapsone. N Engl J Med. 1990;323:776-782.
4. Fralick M, Macdonald EM, Gomes T, et al. Co-trimoxazole and sudden death in patients receiving inhibitors of renin-angiotensin system: population based study. BMJ. 2014;349:g6196.
5. Gilbert SJ, Weiner DE, Gipson DS, et al. National Kidney Foundation’s Primer on Kidney Diseases. Philadelphia, PA: Elsevier; 2014.
6. Muriithi AK, Leung N, Valeri AM, et al. Biopsy-proven acute interstitial nephritis, 1993-2011: a case series. Am J Kidney Dis. 2014;64(4):558-566.
7. Blank ML, Parkin L, Paul C, Herbison P. A nationwide nested case-control study indicates an increased risk of acute interstitial nephritis with proton pump inhibitor use. Kidney Int. 2014;86(4):837-844.
8. Klepser DG, Collier DS, Cochran GL. Proton pump inhibitors and acute kidney injury: a nested case-control study. BMC Nephrology. 2013;14:150.