A certain commonly prescribed antibiotic can be dangerous to elderly patients already taking popular heart drugs, new Canadian research finds.

Taking trimethoprim-sulfamethoxazole (TMP-SMX) on top of two kinds of heart drugs -- ACEIs or ARBs -- leads to an almost seven-fold increased risk of hospitalization, compared with other antibiotics, the study found.

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

Diovan (valsartan) and Atacand (candesartan) are two of the biggest selling ARB medicines. Common ACE inhibitors include ramipril (Altace), benazepril, enalapril and lisinopril.

TMP-SMX, meanwhile, is commonly prescribed to seniors for treating urinary tract infections.

Researchers from the Institute for Clinical Evaluative Sciences found that elderly patients who took TMP-SMX along with heart drugs can develop abnormally high levels of potassium in the blood, a condition called hyperkalemia. Hyperkalemia is potentially dangerous because it can lead to heart rhythm disturbances.

"Serious hyperkalemia can cause sudden death and is one of the most dangerous complications of therapy with ACEIs or ARBs," the authors write in their study, published in the Archives of Internal Medicine.

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

The researchers note that TMP-SMX remains a popular antibiotic, with more than 107,000 prescriptions written for seniors in Ontario in 2007 alone. But the authors say that given their findings, doctors should consider other antibiotics if their patients are taking the heart drugs studied.

"Our study provides evidence of a potentially dangerous interaction between some very commonly used drugs," the study's lead author, pharmacist Tony Antoniou of Toronto's St ,Michael's Hospital, said in a news release.

"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."