Aspirin Dose Do's and Don'ts

You're just had knee surgery and your doctor tells you to get up and walk around as soon as possible. Why?

An aspirin a day has gained the reputation of being essential to good health; a daily aspirin can help reduce the risk of heart attack and stroke. But the dose each person takes varies between 81 mg (the amount in one children's aspirin) and 325 mg (the amount in one adult aspirin). There is some risk associated with aspirin, so the key lies in knowing which dose is best. Research may be getting closer to an answer.

Risky Business

By inhibiting the cyclooxygenase (COX) enzyme, aspirin blocks the production of thromboxane, a substance in blood platelets that helps them stick together. This helps prevent the formation of heart attack-and stroke-causing blood clots, but it also increases the risk of bleeding.

In the stomach, aspirin exerts a different effect and has the potential to weaken the protective barrier between the stomach's delicate tissues and caustic gastric acids, which may in turn lead to fatal ulcers.

Aspirin use has also been linked to hemorrhagic stroke, which occurs when the wall of a weakened blood vessel bursts. In fact, a recent study in The Lancet found that this type of stroke, though less common than an ischemic stroke caused by a blood clot, has increased sevenfold in the past 25 years, particularly among people over 75 who took aspirin or other blood thinners such as Plavix (clopidogre).

This sounds scary, but the protective effects of aspirin therapy are so great and the risks are so relatively small fro most people, that it remains an essential part of preventing cardiovascular complications.

The Right Dose

A recent study in the Journal of the American Medical Association (JAMA) analyzed the findings from 11 major aspirin studies and found that a daily 325-mg adult aspirin was no better at protecting people with cardiovascular disease (CVD) from having a heart attack or stroke over a period of 2.5 years than a children's aspirin dosage of 81 mg daily.

Most important, a small percentage taking higher doses were more likely to experience gastrointestinal bleeding. The authors' conclusion: For most people, 81 mg a day is recommended. In Europe, a 75-mg dose is available and is a reasonable choice.

Ty Gluckman, M.D., an adjunct faculty member at the Johns Hopkins Ciccarone Preventive Cardiology Center, generally concurs but stresses that an exception should be considered for enteric-coated aspirin.

"A number of previous studies that evaluated aspirin's effects on cardiovascular outcomes have used aspirin that was not coated. Most aspirin used in the United States today is enteric coated. Because it's still not clear how much enteric coating affects how well or how fast aspirin is absorbed and used by the body, I have real concerns about using 81 mg of aspirin in its coated form," says Dr. Gluckman.

Enteric coating protects the stomach, but it may also diminish the anti-clotting power of aspirin. Therefore, if you are taking enteric-coated aspirin for the prevention of heart attack or stroke, talk to your doctor about raising your daily dosage from 81 mg to 162 mg daily. Otherwise, Dr. Gluckman says, "Take 81 mg of noncoated aspirin." (Remember, upping the dosage may not be a good idea if you are at high risk for bleeding.)

Be aware that adult 81-mg aspirin is often sold at higher prices than adult 325-mg aspirin. While store-brand children's aspirin is probably the least expensive, another option is to take half of a noncoated adult aspirin every other day. But Dr. Gluckman says, "Even though some of the early prevention trials used a strategy of dosing aspirin every other day, I think people are more likely to remember to take their aspirin if it is dosed daily."

One final note: Aspirin dosages greater than 81 mg daily are currently recommended for people treated with a percutaneous coronary intervention (PCI; e.g., angioplasty, stenting) and in emergency situations.

If someone is having a heart attack, Dr. Gluckman recommends between 162 mg and 325 mg of a chewable aspirin — enteric-coated aspirin is not recommended, as the coating delays the time it takes for the aspirin to get into the bloodstream.

End of Story?

The JAMA study analyzed data from already-completed studies; it was not a large clinical trial designed to test preventive effects of varied aspirin doses. A small clinical trial published in Circulation measured the anticlotting ability of different aspirin doses on blood samples taken from 125 volunteers with CVD and, according to Dr. Gluckman, appears to support the recommendation of 81 mg of uncoated aspirin or 162 mg of coated aspirin. The applicability of this study to most aspirin users is somewhat limited, however, because all the aspirin used in the study was noncocated.

The Circulation study does move science closer to finding the best way to measure how well blood platelets respond to aspirin.

There is considerable evidence that people's response to aspirin varies and that this aftects the degree of their protection from heart attack and stroke. Doctors may be able to personalize aspirin does for patients at some point down the road, but there are limited clinical data to support routine testing at this point.

Can Aspirin Prevent Cancer?

Research has hinted that taking high doses of aspirin may prevent cancer. One observational study in the Journal of the National Cancer Institute followed more than 145,000 people for over 5 years. Participants who reported taking 325 mg or more daily were 22% less likely to develop colon cancer and 19% less likely to develop prostate cancer. Aspirin had no effect on eight other types of cancers, including breast cancer.

Data notwithstanding, the United States Preventive Services Task Force advises people at low to average risk for prostate or colon cancer against taking aspirin for cancer prevention — the dangers of bleeding far outweigh the protective benefits and more study is still needed. If you are at high risk for these cancers, ask you doctor whether higher-dose aspirin is right for you.