While daily use of low-dose aspirin has shown benefits in decreasing risk of heart attacks and strokes, and even colorectal cancer in middle-aged people, it was unclear if these benefits extend to older individuals. Now, former Damon Runyon Clinical Investigator Andrew T. Chan, MD, MPH, and his colleagues at Massachusetts General Hospital in Boston and Monash University in Melbourne, Australia, have published surprising new results that older adults with advanced cancer, taking aspirin may increase their risk of tumor growth and early death.

The researchers ran the first clinical trial testing the effect of aspirin specifically in healthy adults without heart disease, dementia or physical disability over the age of 70 years. One group of individuals took daily low-doses of aspirin (100 mg) and the other was given placebo. The study included 19,114 Australian and American participants who were then followed for about five years. The vast majority of the study participants did not take aspirin previously.

Within the clinical trial, 981 participants who were taking aspirin and 952 who were taking placebo developed cancer—not a statistically significant difference. However, aspirin was associated with a 19% higher risk of being diagnosed with cancer that had spread and a 22% higher risk of being diagnosed with stage 4, or advanced, cancer. Also, among participants who were diagnosed with advanced cancer, those taking aspirin had a higher risk of dying during follow-up than those taking placebo.

“Deaths were particularly high among those on aspirin who were diagnosed with advanced solid cancers, suggesting a possible adverse effect of aspirin on the growth of cancers once they have already developed in older adults,” said Dr. Chan. The findings suggest the possibility that aspirin might act differently, at the cellular or molecular level, in older people, which requires further study.

“Although these results suggest that we should be cautious about starting aspirin therapy in otherwise healthy older adults, this does not mean that individuals who are already taking aspirin–particularly if they began taking it at a younger age – should stop their aspirin regimen,” Dr. Chan added.

Read more: National Cancer Institute

The research was published in the Journal of the National Cancer Institute.