CaPP3: A big step forward in preventing bowel cancer for people with Lynch syndrome
Thursday 26 June 2025
On Thursday 26 June 2025, Sir John Burn announced the results of the Cancer Prevention Project 3 (CaPP3) trial, a landmark study changing how we think about aspirin and cancer prevention for people with Lynch syndrome.
What is CaPP3?
CaPP3 is the latest in a series of ground-breaking studies exploring how aspirin can help prevent bowel cancer in people with Lynch syndrome. It builds on the earlier CAPP2 trial, which showed that taking a high dose of aspirin (600 mg daily for at least two years) can significantly lower the risk of developing bowel cancer.
While CAPP2 was a major milestone, it left an important question unanswered: What’s the smallest dose of aspirin needed to maximise the benefits and limit side effects?
This matters because high doses of aspirin can increase the risk of side effects, including:
- Bleeding in the stomach or gut
- Stomach irritation
- Brusing more easily
What did CaPP3 aim to find out?
CaPP3 set out to discover whether a lower daily dose of aspirin (75–100 mg) could reduce bowel cancer risk just as effectively as higher doses (300 mg or 600 mg), but with fewer side effects.
What do the results show?
The results are very encouraging. CaPP3 has found that a low dose of aspirin (75–100 mg daily) is just as effective at reducing the risk of bowel cancer in people with Lynch syndrome as the higher doses tested.
This is great news for people with Lynch syndrome. It means they can still benefit from the protective effect of aspirin, while reducing the risk of possible side effects — and they may find it easier to stick to taking it long-term.
Why does this matter?
Despite strong evidence that aspirin helps lower cancer risk in Lynch syndrome, we know that only around one in four people currently take it every day. There are several reasons for this:
- Many people, and even some healthcare professionals, aren’t aware of the benefits
- Some doctors feel unsure about what dose to recommend and worry about prescribing higher doses
- Understandably, people can be concerned about side effects, especially at higher doses
The CaPP3 results help to tackle these barriers. They:
- confirm that a low dose of aspirin is still effective
- give clearer guidance for doctors on what to prescribe
- offer reassurance to people with Lynch syndrome that they can reduce their cancer risk with less worry about side effects
- support the case for updating national guidance, including the NICE Lynch syndrome guideline (NG151)
What do we want to see happen now?
We want to see health authorities across the UK to update formularies and national guidelines, such as NICE NG151, to reflect these results and encourage more doctors to prescribe aspirin for people with Lynch syndrome.
Thinking about taking aspirin?
If you have Lynch syndrome, taking aspirin daily could help reduce your risk of bowel cancer. But it’s really important not to start aspirin on your own. Talk to your GP or specialist first.
They can explain the benefits and risks for you personally, discuss ways to protect your stomach (for example, by testing for Helicobacter pylori or prescribing a proton pump inhibitor like omeprazole) and check whether aspirin is safe alongside any other medication you take.
Questions to ask your doctor:
- Should I take aspirin to help lower my cancer risk?
- What dose would you recommend for me?
- What side effects should I look out for?
- Can I do anything to protect my stomach?
- Will aspirin affect my other medications or health conditions?
- Can I get aspirin on prescription, or do I need to buy it myself?
Find out more
You can read more about our research with Sir John Burn here.
You may also find these pages helpful:
- The Lynch Choices website provides information to help people with Lynch syndrome make decisions, such as taking aspirin and having surgery
- Lynch syndrome UK provide information and support for people with Lynch syndrome. They also have an online support group with almost 4000 other people with Lynch syndrome
Together, we’re making strides towards a future where fewer people with Lynch syndrome develop bowel cancer — and CaPP3 brings us one step closer.
