Canadian researchers are warning about a potentially dangerous interaction between common cardiovascular medications and a frequently prescribed antibiotic. The study of 300,000 seniors conducted at the Institute for Clinical Evaluative Sciences in Ontario found that angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs) can be a toxic combination with the antibiotic combination trimethoprim-sulfamethoxazole (TMP-SMX), widely prescribed to treat urinary tract infections.

ACEIs, or ACE inhibitor, and ARBs are used by millions of people worldwide to treat high blood pressure, kidney disease and heart failure. Common ?id=15" ACE inhibitor medications include ramipril (Altace), benazepril, enalapril and lisinopril. Diovan and Atacand are two of the most widely-prescribed ARBs.

Both TMP-SMX and cardiovascular drugs tend to increase potassium levels in the blood. Taking the two together put patients at risk of developing potentially life-threatening high potassium levels. Hazardously high potassium levels, known as hyperkalemia, can cause fatal disruptions in heart rhythms, leading to cardiac arrest and death. "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.

"Our study provides evidence of a potentially dangerous interaction between some very commonly used drugs. Physicians and pharmacists need to be aware of the potential for this interaction, and, if possible, select alternative antibiotics that don't interact with ACEIs or ARBs," says the study's lead author, Tony Antoniou. The study showed that patients hospitalized with hyperkalemia were almost seven times more likely to have received a prescription for TMP-SMX in the preceding 14 days compared to other antibiotics. This potentially life-threatening complication can easily be avoided by simply using another type of antibiotic, such as amoxicillin, with patients already using ACEIs or ARBs, stress the researchers.

On average, seniors take about three times more medications than younger people do. The most commonly prescribed drugs are for the treatment of chronic conditions, cardiovascular problems and heart failure, such as ?id=15" high blood pressure medications .

A separate Canadian study found that just under two-thirds of seniors 65 and older were taking five or more prescription drugs on an ongoing basis, and twenty per cent were taking 10 medications or more. For example, a senior with angina (recurring chest pain) who had had a heart attack may be taking five or six different drugs - a calcium channel blocker, an ace inhibitor, a beta blocker, Aspirin, nitroglycerin, and often a diuretic.