What Is the Evidence for Aspirin in Cancer Prevention?

The short answer

Long-term low-dose aspirin (75–100mg/day) reduces colorectal cancer incidence by 30–40% and colorectal cancer mortality by 35–40% after 5–10 years of use. However, the ASPREE trial found that aspirin in healthy older adults increased cancer mortality (possibly by promoting metastasis) and gastrointestinal bleeding without cardiovascular benefit. The US Preventive Services Task Force now recommends against initiating aspirin for primary prevention in adults over 60.

What the evidence actually shows

Rothwell et al. (2010) in The Lancet, pooling data from 8 randomised trials, found that daily aspirin reduced colorectal cancer incidence by 24% after 20 years and colorectal cancer mortality by 35% after 5 years of use. The mechanism involves inhibition of COX-2, which promotes tumour growth. However, the ASPREE trial (McNeil et al., 2018, NEJM), randomising 19,114 healthy older adults to aspirin or placebo, found that aspirin did not reduce cardiovascular events but increased major haemorrhage by 38% and, unexpectedly, increased cancer mortality by 31% — possibly because aspirin promotes growth of already-established cancers.

"Daily aspirin reduced colorectal cancer incidence by 24% and mortality by 35% after 5 years of use, but increased major haemorrhage by 38% in healthy older adults."

— Rothwell et al., The Lancet 2010; McNeil et al., NEJM 2018

What the major health authorities say

The US Preventive Services Task Force (2022) recommends against initiating aspirin for primary prevention of cardiovascular disease in adults aged 60 and older, citing the increased bleeding risk without sufficient cardiovascular benefit. MedlinePlus notes that aspirin is used for pain relief and to prevent blood clots, but that its use for primary prevention should be discussed with a doctor. The NIA recommends against self-prescribing aspirin for prevention without medical advice.

Practical implications

Aspirin should not be started for primary prevention (in people without established cardiovascular disease or prior heart attack/stroke) without medical advice, particularly for adults over 60. For individuals with established cardiovascular disease or prior cardiovascular events, aspirin remains recommended as secondary prevention. For colorectal cancer prevention specifically, aspirin is most beneficial in individuals at high risk (family history, Lynch syndrome) — a decision that should be made with a gastroenterologist or oncologist.

Where reasonable people still disagree

• Whether the increased cancer mortality seen in ASPREE is a real biological effect of aspirin or a statistical artefact — the finding was unexpected and has not been replicated.
• The optimal aspirin dose for cancer prevention — whether 75mg/day is sufficient or whether higher doses are needed for the full anti-cancer effect.
• Whether the cancer prevention benefits of aspirin outweigh the bleeding risks for specific high-risk populations (Lynch syndrome, family history of colorectal cancer).