Studies are conflicting, there’s no consensus on dosage, and we have an alternative way to prevent cancer that is probably more effective.
Though Aspirin is primarily an antiplatelet agent given to patients after a heart attack, at higher doses it can also be anti-inflammatory and treat pain or fever. It’s seldom used for that purpose today because high doses can be irritating to the stomach and it has largely been supplanted by other medications, like ibuprofen (Advil) and naproxen (Aleve). But its anti-inflammatory action has a possible secondary benefit in cancer prevention, especially when it comes to colorectal cancer. Anti-inflammatories suppress the inflammation that can induce polyp formation, though other mechanisms may be at play as well.
Interpreting the sometimes conflicting data on this issue is complicated. Many of the observational studies showed a benefit, but not all the randomized controlled trials have. But randomized trials are expensive and hard to maintain long term. Some of the negative trials may simply not have gone on long enough to demonstrate a cancer reduction.
There’s also no consensus on what dose of Aspirin would be necessary to prevent cancer. Higher doses might be more effective, but would likely come with more side effects. Given the long latency of colon cancer, you might need to take Aspirin for 10 to 15 years to achieve any meaningful cancer reduction and subject yourself to a higher bleeding risk for that entire period without extracting any benefit.
As with cardiovascular disease, the idea of taking Aspirin for cancer prevention is not clear cut. The benefits are small and only seen far into the future, and are probably outweighed by the bleeding risks for most people. That’s why most guidelines do not recommend it, and neither do I.