CaPP3: low-dose aspirin cuts bowel cancer risk in Lynch syndrome

The results from the CaPP3 trial show that a low daily dose of aspirin (75–100 mg) can prevent bowel (colorectal) cancer in people with Lynch syndrome, and does so as effectively as higher doses. Led by Professor Sir John Burn at Newcastle University and funded by Cancer Research UK and Stand Up To Cancer, the findings were presented at the Cancer Research UK Cancer Prevention Research Conference in London (25–27 June 2025).

What the trial tested

CaPP3 followed 1,879 people with Lynch syndrome across the UK, several European countries and Australia for five years. Participants were randomly assigned to take one of three daily aspirin doses: 75–100 mg; 300 mg; or 600 mg, and had regular check-ins. The headline result: fewer bowel cancers across all three groups, with the lowest dose performing just as well as the higher doses. Because low doses are linked to fewer side-effects (like bleeding and stomach irritation), this matters for long-term prevention.

Why this matters

We’ve known from the earlier CaPP2 trial that 600 mg aspirin daily could halve bowel cancer risk in Lynch syndrome. But many clinicians and patients were understandably cautious about taking a higher dose. CaPP3 fills the missing piece by showing that “baby” aspirin works too, paving the way for clearer, lower-dose prescriptions.

Right now, NICE guidance says people with Lynch syndrome should consider daily aspirin to prevent colorectal cancer, typically for more than two years. Yet awareness and comfort with prescribing have lagged: in a UK survey, only 46.7% of GPs aware of Lynch syndrome knew aspirin could reduce risk. CaPP3 provides the evidence needed to support specific low-dose prescribing within NHS care. NICEPMC

What happens next

The research team is engaging with regulators and the British National Formulary (BNF) to update prescribing advice so GPs can recommend low-dose aspirin for eligible patients with Lynch syndrome. That change could make prevention more accessible to the estimated 1 in 400 people in England (about 175,000) who have Lynch syndrome—many not yet diagnosed.

A human story behind the data

The first CaPP3 participant, Nick James from Newcastle, joined the trial in 2014 after learning he had Lynch syndrome. He now has biennial colonoscopies and continues daily aspirin, and has not developed cancer. Stories like Nick’s bring the science to life and highlight how prevention can reduce fear and uncertainty.

How this fits with LSUK resources

At Lynch Syndrome UK, we see CaPP3 as a watershed moment for evidence-based prevention. It complements tools like Lynch Choices, which includes a decision aid to help you weigh up aspirin alongside other options (such as screening and risk-reducing surgery) and prepare for shared decision-making with your clinical team.

What this means for you

If you have Lynch syndrome, these results are encouraging—but aspirin isn’t one-size-fits-all. Whether it’s right for you depends on your medical history and potential side-effects. Please discuss aspirin with your GP, genetics service or specialist before starting or changing any medication, and use trusted decision aids to clarify what matters most to you.

If you missed this year's Lynch Syndrome UK Conference watch the recording of Sir John Burn, CaPP3 Trial Lead, talking about the research results on our YouTube channel.

Sources: Newcastle University press release (24 June 2025); Cancer Research UK coverage of CaPP3; NICE patient decision aid; GP awareness study.