Recent studies suggest a blood transfusion during surgery increases the patient's risk of death, particularly from heart attacks or strokes and of serious illnesses, such as pneumonia and cancer of the lymph glands.
The risk, however, is not linked to contaminated blood infecting them with deadly viruses. Instead, scientists are investigating two possible causes, reports the Daily Mail.
One is that donated blood, instead of boosting a sick person's ability to ward off infection, might leave their immune system unable to resist attacks by bacteria and viruses, according to the Journal of the American College of Surgeons.
The other is that transfusions may trigger inflammation in the blood vessels, increasing the risk of heart attacks and strokes following surgery.
This is because during its 30-day shelf-life, stored blood undergoes key changes that can make it toxic for some recipients.
Researchers at the University of Kentucky in the US found that having a small blood transfusion of just one unit, roughly a pint (473 ml), increased the risk of dying within 30 days by 32 percent.
A pint is the lowest amount patients usually receive. Most need about three pints to help them cope with surgery.
The study looked at 125,000 patients undergoing surgery for a range of conditions, such as a hernia or appendix removal.
The National Blood Service in Britain advises that patients facing surgery should eat iron-rich foods - such as red meat, pulses, green leafy vegetables and nuts or seeds - to reduce the risk of anaemia.
"A shortage of iron can cause anaemia and correctin this pre-operation may reduce the need for a transfusion," it concluded.