Aspirin (acetyl salicylic acid), a nonsteroidal anti-inflammatory drug (NSAID), has been linked to reduced risk of breast cancer and its recurrence. This appears to be a result of aspirin's anti-inflammatory properties or cellular pathways independent of estrogen signaling. The relationship between aspirin use and breast cancer type has not been established.

Aspirin is an NSAID with various activities and side effects

Aspirin is used as a pain killer, fever reducer, and anti-inflammatory. Aspirin also has blood-thinning properties and is used long term in low doses to prevent heart attacks (although preliminary evidence has linked daily aspirin use to increased risk of heart failure). Regular aspirin use can have potentially serious side effects, including gastrointestinal bleeding, anemia, hemorrhagic stroke, and age-related macular degeneration.

Aspirin reduces breast cancer promoting factors

Aspirin reduces breast cancer risk in part by reducing inflammation and inhibiting COX-2 overexpression (NSAIDs act by blocking the production of prostaglandins, of which COX-2 is one). Aspirin also might reduce circulating sex steroid hormone levels that are associated with increased breast cancer risk. Aspirin has been reported to inhibit aromatase activity (in which androgens are converted to estrogens in the body). One study of 740 postmenopausal women in the Nurses' Health Study reported that women who used NSAIDs at least 15 days per month had significantly lower levels of estradiol compared with women with no NSAID use. Frequency of use of aspirin (as well as other NSAIDs and acetaminophen) was found to be inversely associated with concentrations of estradiol, free estradiol, the ratio of estradiol to testosterone, and estrone sulfate.

Aspirin use before diagnosis and risk of breast cancer

While some studies have found no link, other studies have reported that aspirin use is associated with reduced risk of breast cancer, especially hormone receptor positive (ER+/PR+) breast cancer. One meta-analysis of data from 33 previous studies reported that aspirin use was associated with a 14% reduction in risk of breast cancer compared to no use, whereas another meta-analysis reported no reduction in breast cancer risk among aspirin users. Another study reported that aspirin was associated with reduced breast cancer risk in BRCA1 and BRCA2 mutation carriers. A 2020 large prospective French study reported for the first time that while there was no statistically significant association between NSAID use and risk of breast cancer among postmenopausal women, those who also used proton pump inhibitors (PPIs) had lower breast cancer risk.

Aspirin use after diagnosis might reduce risk of recurrence

While aspirin use before a diagnosis of breast cancer might not reduce the risk of breast cancer recurrence, aspirin use after diagnosis appears to reduce risk of recurrence.
One major Danish 2023 study found that low-dose aspirin use was associated with a reduced risk of recurrence in the five, ten, and 15 years after breast cancer diagnosis. The effect was highest in the early years. A large U.S. 2023 study reported that regular aspirin use after diagnosis of nonmetastatic breast cancer was associated with improved long-term survival during up to 32 years of follow-up. Unlike the Danish study, the advantage of aspirin use (regarding breast cancer-specific mortality) was found to be stronger over time.
Another 2023 study found that aspirin use during remission after initial treatment for inflammatory breast cancer (IBC) was associated with significantly improved distant metastasis-free survival and overall survival.
A 2022 study found that women who were unable to achieve pathologic complete response after neoadjuvant chemotherapy had improved outcomes with aspirin use started during the remission period compared to non-aspirin users. Another 2022 study found that regular aspirin use after diagnosis was linked to lower risk of breast cancer-specific mortality, an association that did not differ by breast cancer type.
Another study reported that aspirin use was associated with improved survival in women with stage II and stage III triple negative (ER-/PR-/HER2-) breast cancer. On the other hand, a prospective randomized controlled trial found no breast cancer-free survival benefit in survivors who took 300 mg aspirin daily.
Preliminary evidence suggests that aspirin might enhance the effectiveness of both radiation treatment and tamoxifen. However, note that aspirin should not be used during chemotherapy since it has the potential to interfere with its treatment effects in some cases.

Relationship between aspirin use and breast cancer type is unclear

Studies that have examined the association between aspirin use and breast cancer risk by hormone receptor status have produced conflicting results. The reduction in circulating hormones described above suggests that aspirin use should lower the incidence of estrogen receptor positive (ER+) tumors preferentially. However, one large prospective study that examined this question did not find a significant variation by ER status and another reported an insignificant reduction in ER+ risk combined with an increase in the risk of ER- breast cancer.

Aspirin has more favorable profile than other OTC pain killers

For those who can tolerate its side effects, aspirin is a better choice for pain relief and fever reduction than other over-the-counter pain relievers, including acetaminophen, ibuprofen, and naproxen (Aleve). Most studies have reported that acetaminophen (Tylenol), which is not an NSAID, is not associated with risk of breast cancer or its recurrence. Use of ibuprofen (Advil, Motrin) has been found to be associated with increased risk of breast cancer. Naproxen (Aleve) appears to have either a small or no effect in preventing breast cancer. Note that naproxen should not be taken during Adriamycin (doxorubicin) chemotherapy since it has been shown to increase Adriamycin-induced heart damage (cardiomyopathy).

Food sources of salicylic acid

Aspirin is metabolized into salicylic acid in the body. A wide variety of spices, fruits and other foods contain salicylic acid, although the concentrations are low for the most part. The average daily intake of salicylic acid from foods has been estimated at 0 to 5 mg. Note that drying concentrates the salicylic acid levels of a food. Also note that those with salicylic acid sensitivity should limit or avoid these foods. Below are foods that are good sources of salicylic acid, in addition to having been linked to reduced breast cancer risk:
One study examining the associations between consuming meat, aspirin use and breast cancer found that breast cancer risk was elevated among women who ate meat, especially flame broiled meat, and that aspirin use reduced the meat consumption-related increase in breast cancer risk.
The bark of the white willow tree (Salix alba) contains salicin, which is similar to aspirin. However, white willow bark supplements can be a source of cadmium exposure, due to uptake of cadmium in the soil by the tree.

Bottom line

It is not prudent to attempt to use aspirin as a treatment for breast cancer, partially because effective dosages are far from established and also because of aspirin's potentially serious side effects. Efforts are underway to develop compounds with aspirin's chemopreventive effects without its side effects. In the mean time, aspirin appears to be the best choice for short-term relief from pain and inflammation compared to other commonly-available pain killers.
Below are links to recent studies on this topic. For a more complete list of studies, please click on aspirin.