Taking ibuprofen or other common painkillers for only a week increases the risk of a heart attack, research suggests.
Data from nearly 450,000 patients has linked five forms of painkillers - ibuprofen, celecoxib, diclofenac, naproxen, and rofecoxib - to heart problems.
People who take strong doses of the drugs - called non-steroidal anti-inflammatory drugs (NSAIDs) - are the most at risk, the Canadian researchers found. And the risk starts to rise after only a week of starting the painkillers, the Daily Mail reported.
In relative terms, the risk of a heart attack rose by between a fifth and a half compared to not taking any painkillers, the team calculated.
The researchers from the University of Montreal stressed that because most people have only a small risk of a heart attack to start with, the absolute risk of an attack directly contributed to taking NSAIDs is only about 1 per cent a year. But they said doctors should consider alternative painkillers.
Writing in the British Medical Journal, the researchers said: "Given that the onset of risk of acute [heart attack] occurred in the first week and appeared greatest in the first month of treatment with higher doses, prescribers should consider weighing the risks and benefits of NSAIDs before instituting treatment, particularly for higher doses."
They said for ibuprofen in particular, taking a high dose was especially risky.
"Use for eight to 30 days at a high dose was particularly harmful for ibuprofen (more than 1200 mg/day)," they wrote. This is well within the maximum recommended dose for adults, which is 400mg three or four times a daily - a total of up to 1,600mg.
They found patients who took ibuprofen for a week had a 48 per cent increased relative risk of a heart attack, those who took celecoxib saw a 24 per cent increase, diclofenac 50 per cent, naproxen 53 per cent and refecoxib 58 per cent. For those who continued taking ibuprofen for up to a month, the increase in risk went up to 75 per cent.
Researchers suspect the drugs may cause arteries to constrict, increase fluid retention and raise blood pressure. Alternative theories include the possibility that they encourage the clumping of platelets and formation of blood clots.
The researchers carried out a "meta-analysis" of all previous studies on the subject, combining the results to make a study of 446,763 people, of whom 61,460 had suffered a heart attack.
The authors said it was the largest investigation into the topic ever conducted.
Dr Mike Knapton, associate medical director at the British Heart Foundation, said: "This large-scale study worryingly highlights just how quickly you become at risk of having a heart attack after starting NSAIDs.
"Whether you are being prescribed painkillers like ibuprofen, or buying them over the counter, people must be made aware of the risk and alternative medication should be considered where appropriate."
Professor Kevin McConway, of the Open University, added: "This new study has helped persuade me that there is probably a real association between taking these painkillers and heart attacks.
"But, despite the large numbers of patients involved, some aspects do still remain pretty unclear.
"It remains possible that the painkillers aren't actually the cause of the extra heart attacks.
"We've got to remember that all drugs have side effects, and that people aren't prescribed these painkillers for fun, but to deal with a real pain problem."
John Smith, chief executive of the Proprietary Association of Great Britain, the UK trade association representing manufacturers of over-the-counter medicines, said: "People taking over-the-counter NSAIDs should not be concerned by this research if they are taking the medicine occasionally for short periods and according to the on-pack instructions.
"The study also showed that after patients had taken their last prescribed dose of an NSAID, their risk of having a heart attack then decreased over time back to normal levels of risk."