TORONTO (Canadian Press) -- A study has found that giving a certain antibiotic to elderly patients already taking commonly prescribed cardiovascular drugs can be dangerous.

The study found that taking trimethoprim-sulfamethoxazole (TMP-SMX) on top of ACEIs or ARBs leads to an almost seven-fold increased risk of hospitalization compared with other antibiotics.

TMP-SMX taken along with one of the other two classes of drugs can cause abnormally high levels of potassium in the blood, a condition called hyperkalemia.

ACEIs, which stands for angiotensin converting enzyme inhibitors, and ARBs -- angiotensin receptor blockers -- are used by millions of people worldwide to treat high blood pressure, kidney disease and heart failure.

Hyperkalemia is a potentially dangerous side-effect of these drugs because it can lead to heart rhythm disturbances.

The 1994-2008 study of Ontarians aged 66 years or older found almost 12 per cent of patients received at least one prescription for TMP-SMX while on ACEI or ARB therapy.

"Our study provides evidence of a potentially dangerous interaction between some very commonly used drugs," said lead author Tony Antoniou of the Institute for Clinical Evaluative Sciences.

"Physicians and pharmacists need to be aware of the potential for this interaction and, if possible, select alternative antibiotics that don't interact with ACEI or ARBs."

Copyright The Canadian Press, 2010