Older people taking daily low-dose aspirin have a 20% higher risk of developing anemia, new research shows.

The study, published Monday in the Annals of Internal Medicine, examined hemoglobin concentrations among more than 19,000 healthy adults in the U.S. and Australia who were 65 and older.

Low levels of hemoglobin, an iron-rich protein in red blood cells that carry oxygen throughout the body, can lead to anemia, which is common in older people and can cause fatigue, a fast or irregular heartbeat, headaches, chest pain, and pounding or whooshing sounds in the ear. It can also worsen conditions such as congestive heart failure, cognitive impairment, and depression in people 65 and older.

“We knew from large clinical trials, including our … trial, that daily low-dose aspirin increased the risk of clinically significant bleeding,” said Zoe McQuilten, MBBS, Ph.D., a hematologist at Monash University in Australia and lead author on the study. “From our study, we found that low-dose aspirin also increased the risk of anemia during the trial, and this was most likely due to bleeding that was not clinically apparent.”

The U.S. Preventive Services Task Force changed its recommendation on aspirin as a primary prevention of cardiovascular disease in 2022, recommending against starting low-dose aspirin for adults 60 years or older. For adults ages 40 to 59 with a 10% or greater 10-year risk for cardiovascular disease, the agency recommends patients and health care providers make the decision to start low-dose aspirin use on a case-by-case basis, as the overall benefit is small.

McQuilten said doctors can’t find the cause of many anemia cases. One study published in the Journal of the American Geriatrics Society in 2021 found about one-third of anemia cases did not have a clear cause.

About 50% of people involved in the latest study took aspirin for prevention from 2011 to 2018. That number likely dropped after changes to the guidelines in 2022, according to McQuilten, but may have continued long-term among older patients. The researchers also examined ferritin levels, which serve as a proxy for iron levels (and can signal anemia at low levels), at the start of the trial and again after 3 years.

People taking aspirin were more likely to have lower blood serum levels of ferritin at the 3-year mark than those who took a placebo. The average decrease in ferritin among people in the study who took aspirin was 11.5% greater than those on a placebo.

The estimated chance of anemia within 5 years was 23.5% in the aspirin group and 20.3% in the placebo group. Overall, aspirin therapy resulted in a 20% increased risk for anemia.

Basil Eldadah, MD, Ph.D., a supervisory medical officer at the U.S. National Institute on Aging, said the findings should encourage healthcare providers to discuss the need for taking aspirin with their patients.

“For somebody who’s taking aspirin and who’s older, and it’s not for an indication like cardiovascular disease, consider seriously whether that’s the best treatment option,” said Eldadah, who was not involved in the study.

The study did not examine the consequences of anemia on participants, which he said could be the subject of future research. The researchers said one limitation was that it was not clear if anemia was enough to cause symptoms that affected the quality of life of the people in the study, or if unknown bleeding caused the anemia.

The researchers also did not document whether patients saw their regular doctors and received treatment for anemia during the trial.

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