Christopher Labos: Why Aspirin isn’t routinely prescribed for cancer prevention

Studies are conflicting, there’s no consensus on dosage, and we have an alternative way to prevent cancer that is probably more effective.

Published Jul 17, 2024 • Last updated 50 minutes ago • 3 minute read

As with cardiovascular disease, the idea of taking Aspirin for cancer prevention is not clear cut, Christopher Labos writes. Photo: Postmedia files

In my last column, I tried to make the case that people without a history of cardiovascular disease shouldn’t be taking baby Aspirin because the cardiovascular benefits are largely counterbalanced by the increased risk of bleeding. But one reader contacted me questioning why I had not mentioned the cancer benefit, since he had spent years taking a daily Aspirin for expressly that purpose.

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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.

At this point, you might ask why these medications are not routinely recommended to the general population as a way of reducing the second-most common cause of cancer death in Canada. The problem is that non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and naproxen come with some significant drawbacks. With regular use, they raise blood pressure, cause stomach ulcers and increase the risk of cardiovascular disease overall. Prescribing them to the general population for long-term use would be significantly dangerous and cause a huge number of excess cardiovascular events.

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Hence the interest in Aspirin, which does not have the same negative cardiovascular profile of other NSAIDs and would potentially be much more useful. A summary of human trials does suggest that Aspirin reduces the incidence of colon polyps. But when it comes to cancer prevention, the data is more nuanced. The Physicians’ Health Study randomized more than 22,000 healthy male physicians to 325 mg of Aspirin every other day. During the initial five-year followup, there was no reduction in colorectal cancer; this was confirmed after 12 years of followup as well. A similar study in women, the Women’s Health Study, tested 100 mg of Aspirin every other day and also found no benefit during the initial study. However, the long-term data with 18 years of followup did show a modest reduction in cancer, albeit with more bleeding and stomach ulcers as a trade-off.

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.

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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.

In high-risk patients, perhaps the trade-off between cancer prevention and bleeding would be acceptable. But in a trial of patients with Lynch syndrome (a genetic form of colon cancer), Aspirin showed no benefit, although the trial lasted only four years.

All this nuance must be interpreted with the knowledge that we have an alternative way to prevent cancer that is probably more effective. Regular colorectal cancer screening with stool testing and colonoscopy is a highly effective and probably more cost-effective cancer prevention strategy.

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.

Christopher Labos is a Montreal physician, co-host of the Body of Evidence podcast and author of Does Coffee Cause Cancer?

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