(Medical Xpress) -- People without a history of cardiovascular disease (such as heart attack or stroke) are unlikely to benefit from a regular dose of aspirin, given the associated risk of internal bleeding. This is the finding of the largest study to date into the effects of aspirin in people without established cardiovascular conditions.

Aspirin reduces the risk of clots forming in blood vessels and thereby protects against heart disease and stroke. It is widely used to prevent a repeat heart attack or stroke among people who have already suffered from one of these conditions, known in the medical field as secondary prevention. Many medical experts have also prescribed regular aspirin as a primary prevention technique  a precaution among people without a previous history of heart attack or stroke, but who may be considered at increased risk of these conditions in the future due to the presence of risk factors for heart attacks or strokes.

Researchers from Professor Kausik Rays group at St Georges, University of London investigated the drugs effectiveness in primary prevention and the prevalence of side effects. They also assessed if aspirin had any impact on the risk of death from cancer among people considered at risk of cardiovascular disease.

They analysed data from nine clinical trials involving over 100,000 participants without a history of cardiovascular disease. Half of the participants took aspirin and half took a placebo. The average participant in the aspirin arm of these trials took aspirin for about six years.

The researchers found that although aspirin in conventional daily or alternate day doses reduced the risk of total cardiovascular disease events by 10 per cent, this was largely due to a reduction in non-fatal heart attacks. It did not include a reduction in other cardiovascular disease events including death from heart attack, or fatal or non-fatal stroke.

The study also showed that this benefit was almost entirely offset by a 30 per cent increase in risk of life-threatening or debilitating internal bleeding events. This means that while one cardiovascular disease event was averted for every 120 people treated with aspirin for about six years, one in 73 people suffered from potentially significant bleeding during the same period.

The lead author of this report, Dr. Rao Seshasai, emphasises that people with an established history of heart conditions must not stop taking their medication:

The beneficial effect of aspirin on preventing future cardiovascular disease events in people with established heart attacks or strokes is indisputable. We urge people with these conditions not to discontinue their medication unless advised to do so by their physicians for valid reasons.

However, the benefits of aspirin in those individuals not known to have these conditions are far more modest than previously believed and, in fact, aspirin treatment may potentially result in considerable harm due to major bleeding. Hence, it would be worthwhile to review the existing recommendations, such as the US Preventive Services Task Force guidelines and the Joint British Societies Guidelines, for the use of this agent in low-risk populations, and consider aspirin treatment more selectively on a case-by-case basis.

By concurrently investigating the effects that aspirin had on death from cancer in the same population, the researchers found that, contrary to some recent reports, aspirin did not reduce the risk of death from all cancers.

Dr. Seshasai added that: There is an enormous interest in understanding the role of aspirin in cancer prevention. No evidence of benefit was found in the studies reviewed, but more research is needed given these were only of six years in duration.

The results of this study are published in the Archives of Internal Medicine online (9 January 2012).

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Provided by St. George's University of London