Common Antibiotic Increases Risk of Hyperkalemia in Older Adults

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A new Canadian study warns that older adults who take the widely prescribed antibiotic TMP-SMX, a combination of trimethoprim and sulfamethoxazole (brand name Bactrim, Septra, or Sulfatrim) used to treat urinary tract infections, are at an increased risk of developing potentially life-threatening high potassium levels.

High blood potassium is clinically known as hyperkalemia and can result in an irregular heartbeat or slow, weak, or absent pulse. Hospitalization and close monitoring are required. Treatment consists of potassium lowering medications, diuretics, or dialysis.

The researchers examined the medical records of 300,000 older adults in the Canadian province of Ontario who were taking beta blockers, drugs used for the management of cardiac arrhythmias, and were hospitalized with one of five antibiotics.

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The risk of severe hyperkalemia was five times higher in patients prescribed TMP-SMX compared to those prescribed amoxicillin, another popular antibiotic treatment for simple bladder infections.

Trimethoprim is structurally related to the potassium-sparing diuretic amiloride and has been shown to block sodium channels in the distal nephron, which reduces potassium secretion, according to the researchers.

"Hyperkalemia is a potentially deadly adverse drug reaction," Dr. Matthew A. Weir, of London Health Science Center, said in a news release from the American Society of Nephrology. "TMP-SMX can decrease the kidney's ability to remove potassium from the body." Since potassium plays a key role in regulating heartbeat, he said, abnormally high levels of potassium "can cause fatal disturbances in the heart rhythm."

Urinary tract infections are the most common infectious problem among older adults. Doctors may be able to help patients lower their risk by monitoring blood levels of potassium more closely during treatment.

The study was published online July 1 in the Clinical Journal of the American Society of Nephrology.

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