Study: Aspirin Cuts Colon Cancer Long-Term

July 16, 2013

Women randomly assigned to take aspirin have a long-term lower risk of colorectal cancer, a large follow-up study finds. The women were cancer-free (except non-melanoma skin cancers) when the study began. They were randomly divided into 2 groups. One group took low-dose aspirin every other day. The other group took placebo (fake) pills. The study lasted 10 years. By the end, the difference in colorectal cancer rates between the groups was small enough that it could have been caused by chance. But women were urged to continue their treatments. About 33,000 agreed to let researchers keep track of them long-term. That's when researchers started seeing a difference. For the whole 18-year period, colorectal cancer rates were 20% lower in the aspirin group. But the difference for years 10 through 18 was 42%. Women who took aspirin did have more side effects. About 8.3% had stomach or colon bleeding and 7.3% had peptic ulcers. In the placebo group, 7.3% had bleeding and 6.2% had ulcers. The journal Annals of Internal Medicine published the study online. HealthDay News and MedPage Today wrote about it July 15.

What Is the Doctor's Reaction?

Multiple studies have suggested that regular aspirin use may reduce the risk of developing colon and rectal cancer in men and women. But they were observational studies. These types of studies look at what happens when people choose different behaviors or treatments.  They cannot prove a cause and effect. In this case, something else that's different between the people who did and didn't take aspirin might be the real cause.

But it does seem plausible, from a biological perspective, that aspirin might decrease colorectal cancer risk. 

• Aspirin is an anti-inflammatory drug. Long-lasting inflammation does increase the risk of some cancers.
• Aspirin also blocks the effects of cyclooxygenase (COX) enzymes. These are the enzymes that lead to pain and swelling from inflammation. COX enzymes also play a role in helping cells multiply and spread.

What's been missing until now is a clinical trial. This type of study provides the best evidence for a true cause and effect. A clinical trial randomly assigns people who are otherwise similar to follow different treatments or behaviors.

Finally we have a clinical trial that indeed does confirm aspirin's protection against colon cancer for some people. The results reported today are part of the Women's Health Study. The study enrolled nearly 40,000 women beginning in 1993. It ended in 2004.

The women were randomly assigned to take 100 milligrams of aspirin or a fake pill (placebo) every other day. Women were not told what they were taking.

The researchers have continued to follow what has happened to these women even though they no longer are being told to take the pills. During the first 10 years of the study, there was no significant difference in colon cancer between the 2 groups.

The preventive effect of aspirin rose beginning 10 years after the women started taking low-dose aspirin. The effect was most pronounced by the last year of data collection, in 2012. That was 18 years after women started the study.

Most colon cancers start as polyps. But the women taking aspirin had similar rates of polyps as the women taking the placebo pills. So it appears that aspirin doesn't prevent polyps. But it does decrease the risk of polyps becoming cancers.

What Changes Can I Make Now?

It's still too soon to recommend low-dose aspirin for everyone to prevent colon cancer. In this study and almost all others, even low-dose aspirin increased the risk of internal bleeding and peptic ulcers.

However, if your risk of colon cancer is higher than average, it's worth considering prevention with aspirin. Higher risk is found among people with:

• A prior diagnosis of colon polyps
• A family history of polyps or colon cancer
• A family history of Lynch syndrome, a genetic condition that leads to higher rates of several types of cancer
• A history of ulcerative colitis or Crohn's disease

There are other ways you can help prevent colon cancer:

• Get screened with colonoscopy once every 10 years to look for polyps and to have them removed.
• Stay physically active and dedicate time to exercise each day.
• Don't smoke. Use alcohol in moderation or not at all.
• Maintain a healthy body weight.

Although not proven, some other steps may help reduce your colorectal cancer risk. Here's what else you can do:

• Eat a diet rich in fruits and vegetables.
• Get enough vitamin D through sunlight, diet, pills or all of these.
• Choose whole-grain products that provide more fiber.

What Can I Expect Looking to the Future?

Not all colon cancers are the same.  They have genetic differences. Scientists have already found that some colon cancers are sensitive to aspirin and some are not. In the future, you will have a more accurate assessment of your colon cancer risk. This will help you decide whether prevention with aspirin makes sense for you.