Beta blockers are thought to combat surgery-related stress
The use of beta blocker drugs before surgery to cut the risk of heart problems may be counter-productive, a study suggests.
Researchers found patients given the drugs were a third more likely to die within a month of surgery than those given a dummy pill.
Those on the blood pressure-lowering drugs also had double the risk of having a stroke.
The study, by Canada's McMaster University, appears in The Lancet.
Although use of the drugs did reduce the risk of a heart attack following surgery, the researchers concluded that on balance they did more harm than good.
They estimate use of the drugs before surgery may have contributed to at least 800,000 deaths worldwide in the past decade.
Professor Peter Weissberg, medical director of the British Heart Foundation, said the research emphasised the importance of undertaking large clinical trials, rather than assuming that drugs will only do what is expected of them.
"Fortunately, the practice of giving beta blockers to all patients at risk of vascular disease before surgery is not widespread in the UK.
"Also, the results of this trial - of a relatively high dose of a beta blocker given before surgery - do not mean patients stabilised on long term beta blocker therapy will be at increased risk if they have surgery."
Researcher Dr PJ Devereaux said: "There is a real potential that beta blockers are causing serious harm in the surgical setting.
"If my mother was undergoing surgery and given a beta blocker, I would be extremely upset based on this evidence."
Counter to stress
It has long been thought that using beta blockers before major surgery protects the heart against the stress of the procedure.
Surgery often raises levels of stress hormones known as catecholamines, which drive up a person's blood pressure and heart rate.
Beta blockers block the effects of increased catecholamines.
However, the body can also go into a state of shock after surgery during which blood pressure falls. The researchers suspect that beta blockers may amplify this state.
The McMaster team studied more than 8,000 patients in 23 countries who were undergoing major surgery unrelated to the heart, but who had or were at risk of clogged arteries.
Half were given the beta blocker metoprolol two to four hours before and 30 days after their surgery. The other half got a placebo.
The results suggests that the use of beta blockers could prevent 15 out of every 1,000 people with similar risks undergoing similar surgery from having a heart attack.
However, eight more people would die, five more would have a stroke, 53% would have abnormally low blood pressure and 42% would have an abnormally slow heart beat.
Overall, the risk of death rose from 2.3% for the placebo group to 3.1% for those given beta blockers in the 30 days after their operation.