Aspirin use has been associated with a reduced risk of colorectal cancer. However, there are conflicting data on aspirin’s protective effect in BC. It has been postulated that aspirin’s anticancer effects are due to inhibition of cyclooxygenase enzymes, which affect apoptosis, cell proliferation, invasiveness, migration, and downregulation of aromatase, an enzyme associated with estrogen receptor–positive (ER+) BC.

To help clarify the association between use of low-dose aspirin (i.e., 75 mg or 160 mg) and the risk of BC, a large, nationwide population-based cohort study was conducted in Norway. The study involved data from all national registries, which included information on prescription use, cancer diagnoses, demographic data, and cause of death. All women who lived in Norway for at least 6 months were followed, and those aged 50 years and older were included in the study. Women with a history of invasive cancer before the start of follow-up (except for invasive nonmelanoma skin cancer) and those who were enrolled for fewer than 4 years were excluded. Women were followed until a diagnosis of either BC or another form of cancer was made, death, emigration, or administrative censoring on December 31, 2018.

The study included a cohort design to estimate the association between the use of low-dose aspirin and the risk of BC and a nest-case control to categorize low-dose aspirin by duration of use and to assess trends in BC risk. Low-dose aspirin exposure was categorized as never, past, current less than 2 years, 2 to 3.9 years, and 4 or more years. Subgroup analyses were conducted based on ER+ status, molecular subtype, stage, age (50-64.9 years or 65 years or older), and BMI (<25 kg/m2 or >25 kg/m2).

A total of 1,08,629 women were included in the study, with a median follow-up of 11.6 years. Among the significant findings:

• When the data were analyzed by molecular subtype and age, there was a reduction in the total population by 8% for luminal A type BC among current low-dose aspirin users (compared with nonlow-dose aspirin users). Among current low-dose aspirin users aged 65 years and older, there was a 10% reduced risk of luminal A BC and a 5% decreased risk for ER+ BC compared with nonlow-dose aspirin users
• When the data were examined by duration of low-dose aspirin use, molecular subtype, and age, among those who had used low-dose aspirin for 4 or more years, there was a reduction in BC risk in the total population by 6%. Among those who used low-dose aspirin for 4 or more years and who were aged 65 years or older, there was a 9% decrease in BC risk and a 9% reduced risk of ER+ BC. In the total population, there was a 16% decreased risk of luminal A BC among low-dose aspirin users who used the drug for less than 2 years. Among those aged 50 to 64.9 years, there was a 27% decreased risk for luminal B human epidermal growth factor receptor 2+ (HER2+) BC among past low-dose aspirin users. Among those aged 65 years or older who used low-dose aspirin for 4 or more years, there was a 24% decrease in risk for luminal B HER2+ BC
• When the data were evaluated by molecular subtype and BMI, BC risk was reduced by 9% in those with a BMI >25 who were current low-dose aspirin users. Risk of ER+ BC was reduced by 9% for those with a BMI >25 who received low-dose aspirin currently compared with nonusers. Also, among the BMI >25 group, there was a decrease in BC risk of 14% for luminal A BC among current low-dose aspirin users
• When the data were assessed based on duration of aspirin use, molecular subtype, and BMI, among those with a BMI >25, there was a 14% reduction of risk among low-dose aspirin users receiving the drug for 4 or more years. There was also a 14% reduction in risk of ER+ BC for those receiving low-dose aspirin for 4 or more years who had a BMI >25.

The use of regular-strength aspirin was not reported and there is uncertainty whether these findings may be generalizable to a more ethnically diverse population.

The authors concluded that low-dose aspirin was associated with a small reduction in the risk of ER+ BC in women aged 65 years or older and in those who were overweight. Pharmacists should be familiar with these findings so that they can counsel women who may be interested in the use of low-dose aspirin for BC prevention.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.

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