Adults in the study who took aspirin were 20% more likely to be anemic than those who didn’t take it.
CNN  — 

Aspirin is one of the most commonly used medications in the US. Studies show that more than 40% of adults ages 60 or older take an aspirin every day to prevent dangerous blood clots that could lead to a heart attack or stroke.

In recent years, experts have backed away from blanket use of aspirin therapy for all older adults, however, after studies showed that it carried an increased risk of major bleeding that most likely outweighed any benefit in preventing first heart attacks or strokes. However, it’s still recommended in some cases for people who have had a heart attack or stroke, to prevent another.

Because aspirin can contribute to the danger of big bleeds like aneurysms, researchers wanted to know whether it might also be a factor in more subtle blood loss: the kind that may lead to anemia, or reduced oxygen in the blood.

Anemia is another big problem in the elderly, though perhaps underappreciated compared with heart attacks and strokes. Studies show that 30% of adults 75 and older worldwide are anemic, and anemia is generally tied to worse health – including fatigue, memory and thinking trouble, depression and an increased risk of death.

A study published Monday in the Annals of Internal Medicine followed more than 18,000 adults who were 65 and older from the US and Australia. Half took 100 milligrams of aspirin a day – a low dose – while the other half took a dummy pill. The researchers followed them for about five years. Study participants had yearly doctors visits and blood tests for hemoglobin and ferritin, a protein in blood cells that stores iron.

They saw a small but clear difference. Adults who took aspirin were 20% more likely to be anemic than those who didn’t take it. Based on their results, the researchers estimated that 24% of seniors in the daily aspirin group would develop anemia within five years, compared with 20% of those in the placebo group.

Those on aspirin regimens also had slightly lower levels of hemoglobulin and ferritin, which help blood cells carry oxygen.

The difference remained even when the researchers adjusted their data to account for cancer and for major bleeding events during the study, and for other differences between the participants like age, sex, diabetes, kidney disease and the use of nonsteroidal anti-inflammatory medications, or NSAIDs.

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The study didn’t look at how aspirin may be contributing to anemia, but the authors have an idea about how it might happen. Aspirin makes it harder for blood to clot because it keeps platelets from sticking together. It also blocks and enzyme called Cox-1, which is important for the maintenance of the lining of the stomach and intestines. With this protective barrier damaged, it’s easier for small amounts of blood to leak out of the gut over time, eventually causing anemia.

The researchers wrote that because they saw this effect across many different groups, regardless of their underlying health, it’s likely to be a bigger concern for people who have other risks for anemia, such inflammatory diseases like arthritis or chronic renal insufficiency.

They say doctors should consider more closely monitoring their patients’ hemoglobin levels if they have multiple risk factors, including aspirin use.