The use of aspirin, a nonsteroidal anti-inflammatory drug (NSAID), is associated with reduced risk of breast cancer. This appear to be a result of its anti-inflammatory properties or through cellular pathways independent of estrogen or progesterone signaling. The relationship between aspirin use and breast cancer type is unclear.
Neither pre-diagnostic nor post-diagnosis aspirin use have been shown convincingly to influence recurrence. Aspirin use after a diagnosis of breast cancer has been linked to lower risk of relapse in some studies, especially in overweight and obese women. However, one 2020 study reported that use of aspirin among older adults with cancer was associated with increased metastasis and worse survival, suggesting that aspirin use might accelerate the progression of cancer.
Aspirin's anticancer effects are not shared by other other-the-counter pain killers, some of which might actually increase the risk of breast cancer. Some of the potential benefits of regular aspirin use may be obtained through food sources of salicylic acid.

Aspirin is an NSAID with various activities and side effects

Aspirin (acetyl salicylic acid) 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. Regular aspirin use has potentially serious side effects, including gastrointestinal bleeding, hemorrhagic stroke, and age-related macular degeneration.

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. 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 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.

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:
  • A large prospective study of 26,580 postmenopausal women found that women who regularly consumed aspirin had a 20% lower risk of breast cancer compared to never users of aspirin. There was also evidence of a dosage trend: lower risk of breast cancer was found with increasing frequency of aspirin use. Similar inverse associations between breast cancer risk and regular aspirin use were found for ER+ (23% risk reduction), ER- (22%), PR+ (21%), and PR- (27%) disease. In other words, the lower risk of postmenopausal breast cancer associated with aspirin use did not vary by ER status.
  • A prospective study that included 114,460 women aged 22 to 85 in the California Teachers Study cohort reported that regular use (more than once a week) of aspirin was not associated with risk of breast cancer. Long-term (at least five years) daily aspirin users had a 20% reduced risk of hormone receptor positive (ER+/PR+) breast cancer, but this result was not statistically significant. On the other hand, a statistically significantly 1.8-fold increased risk of ER-/PR- breast cancer was found among those with long-term daily use of aspirin.

Aspirin use after diagnosis might influence risk of recurrence

Aspirin use before a diagnosis of breast cancer may not reduce the risk of breast cancer recurrence. Aspirin use after diagnosis appears to reduce risk of recurrence in some women, especially those who are overweight or obese, however not all studies have found a risk-reduction effect. One 2015 study that examined outcomes only for women who started taking aspirin regularly after diagnosis found no reduction in the risk of breast cancer-specific death among aspirin users. A 2016 study reported that aspirin use was associated with improved survival in women with stage II and stage III triple negative breast cancer. However, as noted above, a 2020 study reported a link between aspirin use and reduced survival among older cancer survivors.
Preliminary evidence suggests that aspirin might enhance the effectiveness of tamoxifen. Note that aspirin should not be used during chemotherapy since it has the potential to interfere with its treatment effects.

Aspirin has more favorable anti-cancer 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 may be too soon 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.