BBC News reports that “low-dose aspirin should NOT routinely be used to prevent heart attacks and strokes,” according to research published in the Lancet.  So, if you are on a daily aspirin for primary prevention of a heart attack or stroke, should you stop taking it?
More Information:
The answer is not an easy one and I recommend you not make the decision alone – but, be sure to talk to your doctor about your individual risks and benefits of taking a daily aspirin (after being sure that he or she is up to date on this new information).
Professor Colin Baigent, the lead researcher for the study, told BBC News, “We don’t have good evidence that, for healthy people, the benefits of long-term aspirin exceed the risks by an appropriate margin.”  
WebMD pointed out that investigators “conducted an analysis” that “included six high-quality primary-prevention studies with a combined enrollment of 95,000 low-to-average-risk people and 16 secondary-prevention studies with a combined enrollment of 17,000 high-risk patients.”
The UK’s Daily Mail explains that the researchers “found that healthy people who take aspirin reduced their already small risk of heart attack or stroke by 12 percent, while the small risk of internal bleeding increased by a third.” 
According to the Daily Mail, “this means there were five fewer non-fatal heart attacks for every 10,000 people treated, but this was offset by a comparable increase in bleeding – one extra stroke and three cases of stomach bleeding per 10,000 people treated.” 
Meanwhile, “in the secondary prevention studies – where patients were taking aspirin to prevent a repeat attack – aspirin reduced the chances of serious vascular events by about one-fifth and this benefit clearly outweighed the small risk of bleeding.” 
Based on incremental cost effectiveness, they recommended aspirin for the following:

• Men age 50 to 59 who are at five times the average cardiovascular risk
• Men age 60 to 69, who have at least twice the average cardiovascular risk and women in the same age range with at least five times the average cardiovascular risk
• All men age 70 to 79 regardless of risk and women in the same age range with at least double the average cardiovascular risk

MedPage Today also covered the story and told doctors, “Explain to interested patients that primary prevention attempts to avoid cardiovascular events in people who do not have heart disease, whereas secondary prevention is begun after someone has a coronary event.”
MedPage added, “Note that U.S. Preventive Services Task Force and American Heart Association guidelines recommend aspirin for primary prevention in patients at moderately elevated risk for heart disease.”
In the meantime, Prescribers Letter is recommending the following to doctors:

• New recommendations will focus more on AGE to determine who should get aspirin for primary prevention.
• Previous guidelines relied more on RISK calculators … and recommended aspirin for patients with a cardiovascular risk of at least 6% over 10 years.
• In general, the new recommendations recommend low-dose aspirin for men age 45 to 79 … and women age 55 to 79.
• These are the ages where the risk of bleeding is usually offset by aspirin’s cardiovascular benefits.
• Interestingly, the benefits are different for men and for women.
  • For men, the benefit is to prevent an MI.
  • For women, the primary benefit is to prevent an ischemic stroke.
• Of course, patients are even more likely to benefit if they have additional CV risks … smoking, hypertension, dyslipidemia, etc.
• On the other hand, patients may be better off without aspirin if they have additional BLEEDING risks … prior GI ulcers, chronic NSAIDs, etc.
• Don’t give aspirin to patients with additional GI risks unless their CV risk is high enough to outweigh the higher bleeding risk.
• Consider adding a proton pump inhibitor if a patient at high risk for GI bleeding needs to take aspirin.
• Patients 80 or older have a high risk of BOTH cardiovascular disease and GI bleeding. Give aspirin only if these seniors have no additional GI risks.
• Also make sure BP is controlled before starting aspirin to reduce the risk of hemorrhagic stroke.
• Prescribe just 81 mg/day of aspirin. There’s no proof that higher doses work better … plus they can increase bleeding risk.
• Advise patients to take either regular aspirin with food or use enteric-coated aspirin … IF needed to reduce stomach irritation. But explain that this only helps the local effects … neither approach reduces the risk of bleeding.

Last, but not least, here are the current USPSTF guidelines:

• U.S. Preventive Services Task Force. Aspirin for the Prevention of Cardiovascular Disease: Recommendation Statement. AHRQ Publication No. 09-05129-EF-2, March 2009. Agency for Healthcare Research and Quality, Rockville, MD.  (Accessed May 21, 2009).
• U.S. Preventive Services Task Force. Aspirin for the Primary Prevention of Cardiovascular Events: An Update of the Evidence. AHRQ Publication No. 09-05129-EF-4, March 2009. Agency for Healthcare Research and Quality, Rockville, MD. (Accessed May 21, 2009).

And, for doctors:

• Risk Calculators:
• CHD risk estimation tool (for Men) here. 
• Stroke risk estimation tool (for Women) here. 

Number Needed to Treat/Harm (for doctors)
Aspirin use for the primary prevention of cardiovascular disease provides more benefits than harms in men or women whose risk for myocardial infarction or ischemic stroke, respectively, is high enough to outweigh the risk for gastrointestinal hemorrhage. 
In men similar to those enrolled in the RCTs, the number needed to treat to prevent 1 myocardial infarction over 5 years of aspirin use is 118, whereas the number needed to treat to cause 1 major bleeding event is 303 over 5 years of aspirin use and 769 to cause 1 hemorrhagic stroke. 
The balance of benefits and harms varies by coronary heart disease risk and risk for gastrointestinal bleeding. 
For a hypothetical group of 1000 men younger than 60 years with a 6% 10-year baseline risk for myocardial infarction, aspirin use will prevent approximately 19 myocardial infarctions and cause approximately 1 hemorrhagic stroke and 8 major bleeding events. 
The USPSTF concluded with high certainty that the net benefit is substantial in men at increased risk for myocardial infarctions and not at increased risk for serious bleeding.
In women similar to those enrolled in the RCTs, the number needed to treat to prevent 1 ischemic stroke with 5 years of aspirin use is 417, and the number needed to treat to cause 1 major bleeding event is 392 over 5 years of aspirin use. 
The balance of benefits and harms varies by stroke risk and risk for a bleeding event. 
In a hypothetical group of 1000 women younger than 60 years with a 6% 10-year risk for stroke, aspirin use will prevent approximately 10 strokes and cause approximately 4 major bleeding events. 
The estimates of the number of major bleeding events were assumed to be stable within age strata with respect to increases in baseline stroke risk. 
The USPSTF concluded with high certainty that the net benefit is substantial for women at increased risk for stroke and not at increased risk for serious bleeding.