Red blood cell transfusions given to heart surgery patients could increase their risk of heart attack or stroke, research suggests.
Most transfusions are not given in life-threatening circumstances
The transfusions are designed to improve the delivery of oxygen to the body's tissues.
But researchers found patients who received a transfusion had a three-fold increase in complications linked to lack of oxygen.
The Bristol Heart Institute study is published in the journal Circulation.
The researchers examined data on over 8,500 patients who underwent cardiac surgery over an eight-year period.
They found the risks associated with transfusion occurred regardless of the patients level of haemoglobin - the oxygen-carrying substance in red blood cells.
Lead researcher Dr Gavin Murphy said the problems were probably linked to changes in the red blood cells which took place while they were in storage, and which affected their capacity to carry oxygen.
In addition, toxins can build up in the fluid in which they are stored.
Dr Murphy said there was evidence to suggest that the cells suppress the immune system, but also increase the risk of potentially damaging inflammation.
However, it was very difficult to pinpoint their effect in patients who were usually already extremely sick.
Professor Peter Weissberg is medical director of the British Heart Foundation, which funded the study.
He said: "Red blood cells carry oxygen around the body to supply vital organs.
"Not unreasonably therefore, heart surgeons have assumed that patients who have low red blood cell counts after surgery - as a result of blood loss during or shortly after surgery - would benefit from a 'top up' transfusion of donated red blood cells.
"This study shows the importance of putting such widespread beliefs to the test since it suggests that such transfusions may cause more problems than they solve."
Professor Weissburg said the results would help to make heart surgery safer.
In the UK over half of all heart surgery patients are given blood.
However, only about 3% of these transfusions are given because of life-threatening bleeding.
The remainder are usually given on the basis of a low haemoglobin level, regardless of whether the patient has physical symptoms to suggest they need blood.
The researchers now intend to carry out a larger study to see if changing transfusion guidelines could improve patient outcomes.
For the time being it is suggested that surgeons think twice before giving their patients a transfusion.
More research is needed to find out how red cell transfusions may affect immunity or tissue oxygenation to cause these harmful effects, and to determine how stored donor blood products may be made safer prior to transfusion.