We Were Wrong About Aspirin (New Evidence)

Scientific Concepts, Discoveries, and Nature/Medical Phenomena

Primary vs. Secondary Prevention with Aspirin

• Primary prevention: Taking aspirin in people without prior cardiovascular events to prevent future disease. (This is the video’s focus for cancer outcomes.)
• Secondary prevention: Taking aspirin after cardiovascular events (e.g., heart attack, stroke, stent).
  • Stopping can be dangerous, and this is treated as a different evidence base.

Aspirin and Colorectal/GI Cancer Risk: Observational to Randomized Evidence

1988 Observational Finding (Melbourne)

• A case-control study reported that people with colorectal cancer used less aspirin than controls.
• After adjustments, aspirin use showed a protective association across colon/rectal cancer and across sexes.

Limitation of Observational Studies

• Confounding/selection may explain associations:
  • People who take aspirin may differ systematically from those who don’t.
• Therefore, causality is unclear.

Randomized Evidence (Meta-analysis of Cardiovascular Trials)

• Peter Rothwell (Oxford) combined cancer outcomes hidden within large heart/stroke trials:
  • Found about a 21% reduction in cancer deaths overall.
  • Benefits appeared to increase with longer follow-up.
  • Gastrointestinal cancer deaths showed the clearest signal, reported as >50% reduction after extended time (e.g., ~7.5+ years).

How Guideline Recommendations Changed

• 2016 USPSTF recommendation:
  • Recommended daily low-dose aspirin (age-based), including reasoning tied to colorectal cancer prevention.
• Later evidence undermined the “cancer shield” narrative.

ASPREE Trial (Australia/US): Cancer Incidence vs. Cancer Mortality

Study Design

• Randomized aspirin vs placebo in healthy older adults:
  • Age: ≥70, or ≥65 for Black/Hispanic participants.
• Exclusions included:
  • dementia, disability, and baseline cardiovascular disease.

Key 2018 Finding (Initial Trial Results)

• Aspirin did not prevent first heart attack or stroke.
• Cancer results diverged from Rothwell’s prediction:
  • People on aspirin died more often from cancer.

2026 Follow-up Publication (ASPREE Extension, JAMA Oncology)

• Cancer incidence: reported as identical between aspirin and placebo.
• Colorectal cancer incidence: also identical, with hazard ratio ~1.01.
• Cancer deaths: about 15% higher in the aspirin group.
• After stopping aspirin (over ~4.5 years):
  • The increased cancer mortality signal vanished.
  • Interpretation: harm may be linked to active use.

Interpretation Shift

• Cancer prevention benefit could not be demonstrated.
• Instead, there is a harm signal in this population/timeframe.

Cochrane Review (Updated Evidence Synthesis)

• Pooled 10 randomized trials with >124,000 participants.
• In the 5–10 year window relevant to starting aspirin in older adulthood:
  • Colorectal cancer mortality was reported as ~77% higher with aspirin.
• Certainty was rated low.
  • Possible benefit at ≥15 years was noted, but with very low certainty.

Mechanistic Hypothesis from Cambridge: Platelets → Thromboxane A2 → Immune Suppression

• Cancer metastasis can involve platelets escorting circulating tumor cells.
• Platelets release thromboxane A2, which suppresses T cells (immune cells that kill rogue cancer cells).
• Aspirin’s low-dose effect:
  • reduces platelet production of thromboxane A2.
• Mouse/young-immune-system experiments suggested aspirin could reduce metastasis by “releasing the brakes” on T cells, potentially reducing metastatic deaths.

Reconciling Mechanism vs. Human Trial Outcomes: Proposed Hypotheses

• Immunosenescence: Aging may make T cells less responsive; removing thromboxane-related braking may not help.
• Masking early symptoms: Anti-inflammatory effects might delay cancer detection by dampening early inflammatory symptoms.
• Chronic immune/inflammatory suppression harms tumor surveillance:
  • Inflammation is part of how the body identifies and controls dangerous cells.
  • Long-term suppression could allow established tumors to grow.

Dose/Person Specificity: Lynch Syndrome Trial (CaPP2)

• CaPP2 (10-year follow-up; The Lancet, 2020):
  • Population: Lynch syndrome carriers (genetic risk up to ~60% lifetime colorectal cancer risk).
  • Intervention: 600 mg/day aspirin (higher than typical 100 mg “low-dose”); median age ~45.
  • Result: about a 35% reduction in colorectal cancer.
• Video-implied conclusion:
  • Any cancer prevention effect may depend on:
    • the person (genetic susceptibility),
    • the dose,
    • starting age / immune context, and
    • duration.

Bleeding Risk as an Established Harm

• Aspirin is associated with about a 59% increased risk of serious bleeding (outside the brain; e.g., GI bleeding).
• This bleeding risk was described as high-certainty in a Cochrane review.

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Methods / Study Designs Mentioned

• Meta-analysis of randomized controlled trials (Rothwell)

  • Extracted cancer endpoints from older cardiovascular RCTs.
  • Analyzed 8 RCTs with >25,000 patients.
  • Evaluated cancer death reduction over time on aspirin and with longer follow-up.

• ASPREE (Aspirin in Reducing Events in the Elderly)

  • Randomized, placebo-controlled trial.
  • Population: healthy older adults (≥70; or ≥65 for Black/Hispanic).
  • Exclusions: cardiovascular disease, dementia, disabilities.
  • Intervention: 100 mg daily aspirin vs placebo.
  • Outcomes: cardiovascular events and cancer incidence/mortality (with extended follow-up).

• Cochrane Systematic Review

  • Updated synthesis of 10 RCTs (>124,000 participants).
  • Assessed mortality/incidence outcomes in relevant follow-up windows (5–10 years and ≥15 years).

• CaPP2 (Lynch syndrome prevention trial)

  • Randomized trial in Lynch syndrome carriers.
  • Intervention: 600 mg/day aspirin vs comparison (per CaPP2 protocol).
  • Follow-up: 10 years, reporting colorectal cancer reduction.

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Researchers or Sources Featured

• Peter Rothwell (Oxford neurologist)
• John McNeil (Professor of epidemiology, Monash University) — led ASPREE
• Gabriel Kune (Melbourne surgeon; 1988 case-control study)
• Susan Orchard (Monash; director of ASPREE extension; lead author of the JAMA Oncology follow-up described)
• Cochrane Library / Cochrane authors (systematic review)
• US Preventive Services Task Force (USPSTF)
• Cambridge researchers (mechanism work on thromboxane A2 and T-cell suppression; not individually named in the subtitles)
• ASPREE team (group-level mention; individual members not fully listed)
• CaPP2 researchers (individual authors not named in the subtitles)

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Key Publications / Journals Mentioned

• The Lancet (Rothwell 2010; related follow-ups; CaPP2 2020)
• JAMA Oncology (ASPREE follow-up described as published in early 2026)
• Nature (Cambridge mechanistic metastasis/platelet thromboxane A2 study)
• Cochrane Library (updated randomized evidence synthesis)