The Heart and Stroke Foundation says a commonly prescribed combination of blood pressure drugs should not be taken together.
The organization says taking a so-called ACE inhibitor in combination with an angiotensin receptor blocker increases the risk of sudden cardiac death and of kidney disease and the need for dialysis.
The warning is included in updated guidelines for Canadian physicians on how to manage high blood pressure in their patients.
The foundation says people who are taking a combination of the two types of drugs should see their family doctors as soon as possible to arrange a change in treatment, but should not change what they take without medical help.
As many as 175,000 Canadians with high blood pressure may be currently taking the combination of the two medications.
Heart and Stroke spokesman Dr. Sheldon Tobe said the two types of drugs are each safe and effective - just not together.
A Canadian-led study published last year found combining the drugs did not produce much benefit to patients and, in fact, increased their risk of some adverse reactions.
The blood pressure guidelines were updated by the Canadian Hypertension Education Program.