How much does aspirin actually lower colorectal cancer risk?
The size of the risk reduction depends heavily on who you are. In a 2024 study of over 107,000 people followed for up to 38 years, regular aspirin users had a 10-year cumulative CRC incidence of 1.98% compared to 2.95% for non-users — an absolute risk reduction of about 1% [1]. That means for every 100 people taking aspirin regularly for a decade, roughly one fewer would develop CRC. A nationwide Norwegian study of 2.2 million people found current low-dose aspirin users had a 13% lower risk of CRC overall (hazard ratio 0.87), with a stronger 21% reduction for metastatic CRC [3]. The benefit was greater with longer use: 5+ years of current use gave a 16% lower risk compared to never users [3].
For people who already had colorectal cancer with certain genetic mutations, the effect was even more dramatic. A 2025 randomized trial gave 160 mg of aspirin daily to patients with PIK3CA-mutated stage I–III CRC. Over 3 years, the recurrence rate was 7.7% with aspirin versus 14.1% with placebo — a 51% relative risk reduction [2]. This is one of the strongest pieces of evidence because it comes from a randomized controlled trial, not just an observational study.
Who benefits most — and when should you start?
The benefit is largest for people with unhealthy lifestyles. In the 2024 study, people with the worst lifestyle scores (obese, smokers, low physical activity, poor diet, high alcohol) had a 10-year absolute risk reduction of 1.28% from aspirin — meaning you'd need to treat 78 such people for a decade to prevent one CRC. For those with the healthiest lifestyles, the absolute reduction was only 0.11%, requiring 909 people to treat for one benefit [1]. This suggests lifestyle risk factors can help identify who gets the most out of aspirin.
Age matters a lot. A pooled analysis of two large US cohorts found that regular aspirin use after age 70 was linked to a 20% lower CRC risk — but only among people who had started aspirin before age 70. Those who started aspirin at age 70 or later saw no significant benefit (hazard ratio 0.92, not statistically significant) [4]. A randomized trial in older adults (ASPREE) found no reduction in CRC over 4.7 years of follow-up for people starting aspirin at age 70+ [5]. The US Preventive Services Task Force now recommends against starting aspirin for primary prevention of CRC in adults 60 or older, citing bleeding risks that outweigh the small benefit [9].
Genetics also play a role. In the 2025 trial, patients with PIK3CA mutations saw a 51% reduction in recurrence [2]. But a separate analysis using a polygenic risk score (95 genetic variants) found that even among older adults with high genetic risk, aspirin did not reduce CRC incidence [5]. So genetic testing may help in some cases but not others.
Do other pain relievers work? And does aspirin help after a cancer diagnosis?
Ibuprofen also appears to reduce CRC risk. In a large screening trial, taking 30+ ibuprofen pills per month was linked to a 24% lower risk of developing an adenoma (a precancerous polyp) and a 52% lower risk of advanced adenoma [8]. Both aspirin and ibuprofen were associated with about a 12–19% lower risk of CRC itself [8]. However, ibuprofen carries its own risks (stomach bleeding, kidney issues), so the choice between them should be discussed with a doctor.
For people already diagnosed with CRC, aspirin may improve survival. A study of over 2,600 patients found that long-term regular aspirin use before diagnosis was linked to a 31% lower risk of dying from CRC (hazard ratio 0.69) [7]. Starting aspirin only after diagnosis also showed a 40% lower risk of CRC death compared to never using it [7]. This suggests aspirin might help prevent the spread of micrometastases. A separate review noted that aspirin use before and after diagnosis reduced metastatic spread and increased survival [6].
About These Sources
This answer is built on 9 peer-reviewed studies — published from 2021 to 2025, 3 from 2024 or later, 8 in Q1 journals, collectively cited 578 times — selected as the most relevant from 12 studies that passed quality screening, drawn from 56 papers retrieved from a database of over 500 million.
Sources used in this answer
Aspirin Use and Incidence of Colorectal Cancer According to Lifestyle Risk
In a prospective cohort of 107,655 people, regular aspirin use was associated with a 10-year absolute risk reduction of CRC of 0.97% overall, with the largest reduction (1.28%) in those with the unhealthiest lifestyles and minimal reduction (0.11%) in the healthiest [1].
Low-Dose Aspirin for PI3K-Altered Localized Colorectal Cancer
In a double-blind randomized trial of 626 patients with PI3K-altered stage I–III CRC, 160 mg aspirin daily reduced 3-year recurrence from 14.1% to 7.7% (hazard ratio 0.49) [2].
Low-Dose Aspirin and Prevention of Colorectal Cancer: Evidence From a Nationwide Registry-Based Cohort in Norway
In a nationwide Norwegian cohort of 2.2 million people, current low-dose aspirin use was associated with a 13% lower CRC risk (HR 0.87), with a stronger 21% reduction for metastatic CRC; longer use (≥5 years) gave a 16% lower risk [3].
Aspirin Use and Risk of Colorectal Cancer Among Older Adults
In a pooled analysis of 94,540 adults aged 70+, regular aspirin use was associated with a 20% lower CRC risk (HR 0.80), but only among those who started before age 70; starting at 70+ showed no significant benefit (HR 0.92) [5].
Aspirin and the Risk of Colorectal Cancer According to Genetic Susceptibility among Older Individuals
In the ASPREE randomized trial of 12,609 adults aged 70+, aspirin was not associated with reduced CRC incidence overall (HR 0.94) or within any genetic risk group defined by a 95-variant polygenic risk score [6].
Using aspirin to prevent and treat cancer
A review of observational and experimental studies concluded that regular aspirin use is associated with a consistent 20–30% reduction in cancer incidence and mortality, with stronger evidence for CRC; antiplatelet effects are thought to be key [8].
Associations of Aspirin and Non-Aspirin Non-Steroidal Anti-Inflammatory Drugs With Colorectal Cancer Mortality After Diagnosis
In a prospective study of 2,686 CRC patients, long-term prediagnosis aspirin use (>15 times/month) was associated with a 31% lower CRC-specific mortality (HR 0.69); postdiagnosis use showed a non-significant 18% reduction overall, but starting after diagnosis gave a 40% reduction [9].
Aspirin, ibuprofen, and reduced risk of advanced colorectal adenoma incidence and recurrence and colorectal cancer in the PLCO Cancer Screening Trial
In the PLCO screening trial, aspirin use (≥30 vs <4 pills/month) was associated with a 12% lower CRC risk (HR 0.88) and a 44% lower risk of advanced recurrent adenoma; ibuprofen showed similar or stronger associations [10].
Aspirin Use to Prevent Cardiovascular Disease
The USPSTF recommends against initiating low-dose aspirin for primary prevention of CVD or CRC in adults 60+ (D recommendation) and suggests individualizing the decision for adults aged 40–59 with ≥10% 10-year CVD risk (C recommendation) [12].