Aspirin has been around for more than 100 years. For most of that time, it was known to offer little more than mild pain relief and modest control of inflammation. Aspirin's use as a pain reliever is now almost forgotten, totally eclipsed by its use to prevent cardiovascular disease. More recently, researchers are now uncovering aspirin’s ability to potentially prevent cancer. Given its many benefits, should we all just be taking aspirin from the day we are born?

Aspirin and colorectal cancer

Perhaps the easiest place to start is by focusing on
colorectal cancer.

Researchers who were studying the effect of low-dose aspirin on cardiovascular outcomes also decided to track the rate of certain cancers.

Several studies showed that low dose daily aspirin could significantly reduce the risk of developing a certain type of colon cancer called colorectal adenoma.1,2,3,4

While some intervening clinical trials failed to show a significant association between aspirin and colorectal cancer, the cumulative evidence to date shows that daily aspirin reduces the risk of colon cancer.

More specifically, patients who take daily aspirin for 2.5 years or more had a 31% lower risk of developing colon cancer.5,6,7 People who took aspirin for five or more years, 38% lower risk of colorectal cancer. It is important to note that these cancer prevention studies were performed in otherwise healthy individuals. The dose of aspirin was considered “low dose”, ranging between 75mg and 100 mg daily.

Aspirin and other forms of cancer

Data supporting the use of aspirin to prevent other forms of cancer is rather murky. There have been six large clinical trials to test the effects of aspirin on various cancers and the results have been conflicting. Given what we know at this point, daily low-dose aspirin use significantly reduced all forms of cancer tested during follow-up.10 In people with less than four years of daily aspirin use, there was no discernible preventative effect. However, people who took aspirin for five years or more were 30% less likely to develop any of the cancers studied in the clinical trial. Of note, the Women's Health Study examined the effect every-other-day, low-dose aspirin in a group of over 40,000 women and found no significant relationship between aspirin use and cancer incidence.3 This suggests that people who use aspirin for cancer prevention need to maintain a daily rather than every-other-day, low-dose aspirin regimen.

Balancing prevention with safety

• Aspirin has been available without a prescription for over a century.
• Cancer prevention regimens suggest using “low-dose” aspirin rather than full strength dosages.

When patients and consumers consider these two facts, they often lose sight of the reality that aspirin can cause significant adverse effects, especially when taken every day.

Chief among these adverse events is the risk of bleeding.

Daily aspirin use—even low-dose aspirin—substantially increases the risk of gastrointestinal and intracranial (inside the head) bleeding.

People who take 325 mg of aspirin (full strength) essentially double their risk of having a serious gastrointestinal bleeding event.8

Those taking low-dose aspirin increased their risk of hemorrhage by 55% to 70%.9,10

Tailoring prevention to the patient

The current consensus is that the decision to use aspirin to prevent cancer should be based on the patient's individual circumstances.11,12 Not everyone's risk of cancer is the same; just as not everyone's risk of gastrointestinal bleeding from aspirin is the same. There are clearly instances in which some patients should receive daily aspirin. Consider, for example, people with a genetic disorder called hereditary non-polyposis colorectal cancer have a lifetime risk of developing colon cancer as high as 80%.13 Clearly, the risk of developing colorectal cancer outweighs the risk of side effects from daily aspirin use. In fact, researchers have shown that patients with this genetic disease appear to benefit from much higher doses of daily aspirin (600 mg/day).14 On the other hand, these higher doses would be inappropriate for most people.

Current recommendations

Based on current clinical trial data, routine low-dose aspirin therapy is not recommended in otherwise healthy individuals.

No major organisation, including the American Cancer Society and the Cancer Council of Australia, recommends the use of low dose aspirin to prevent cancer. Therefore, people with no particular hereditary risk of colon cancer or cardiovascular disease should not be taking low-dose aspirin on a daily basis.15

Whereas, certain high-risk groups of patients could benefit from a daily aspirin regimen because those benefits outweigh the possible risks.

Essentially -

• People in their 50s with low risk of gastrointestinal bleeding or peptic ulcer disease may start taking low dose daily aspirin
• People who take a low dose aspirin for cardiovascular protection presumably receive any cancer-preventing effect of taking the drug.
• Even though aspirin is available without a prescription, a low dose aspirin regimen should only be started and maintained under the supervision of a physician.
• Patients who do opt for a low dose aspirin regimen should be vigilant for signs of gastrointestinal bleeding (e.g. bright red blood in the stools, stools that look like coffee grounds) and notify a physician if these should occur.
• In people with moderate risk of gastrointestinal bleeding, medication to reduce gastric acid production (proton-pump inhibitors) may help reduce the risk of side effects from long-term aspirin use.16

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References

1. Logan RF, Grainge MJ, Shepherd VC, Armitage NC, Muir KR. Aspirin and folic acid for the prevention of recurrent colorectal adenomas. Gastroenterology. Jan 2008;134(1):29-38. doi:10.1053/j.gastro.2007.10.014

2. Benamouzig R, Uzzan B, Deyra J, et al. Prevention by daily soluble aspirin of colorectal adenoma recurrence: 4-year results of the APACC randomised trial. Gut. Feb 2012;61(2):255-261. doi:10.1136/gutjnl-2011-300113

3. Sandler RS, Halabi S, Baron JA, et al. A randomized trial of aspirin to prevent colorectal adenomas in patients with previous colorectal cancer. N Engl J Med. Mar 6 2003;348(10):883-890. doi:10.1056/NEJMoa021633

4. Baron JA, Cole BF, Sandler RS, et al. A randomized trial of aspirin to prevent colorectal adenomas. N Engl J Med. Mar 6 2003;348(10):891-899. doi:10.1056/NEJMoa021735

5. Flossmann E, Rothwell PM. Effect of aspirin on long-term risk of colorectal cancer: consistent evidence from randomised and observational studies. Lancet. May 12 2007;369(9573):1603-1613. doi:10.1016/s0140-6736(07)60747-8

6. Peto R, Gray R, Collins R, et al. Randomised trial of prophylactic daily aspirin in British male doctors. Br Med J (Clin Res Ed). Jan 30 1988;296(6618):313-316.

7. Rothwell PM, Wilson M, Elwin CE, et al. Long-term effect of aspirin on colorectal cancer incidence and mortality: 20-year follow-up of five randomised trials. Lancet. Nov 20 2010;376(9754):1741-1750. doi:10.1016/s0140-6736(10)61543-7

8. Weisman SM, Graham DY. Evaluation of the benefits and risks of low-dose aspirin in the secondary prevention of cardiovascular and cerebrovascular events. Arch Intern Med. Oct 28 2002;162(19):2197-2202.

9. De Berardis G, Lucisano G, D'Ettorre A, et al. Association of aspirin use with major bleeding in patients with and without diabetes. JAMA. Jun 6 2012;307(21):2286-2294. doi:10.1001/jama.2012.5034

10. McQuaid KR, Laine L. Systematic review and meta-analysis of adverse events of low-dose aspirin and clopidogrel in randomized controlled trials. Am J Med. Aug 2006;119(8):624-638. doi:10.1016/j.amjmed.2005.10.039

11. Kaiser J. Will an aspirin a day keep cancer away? Science. Sep 21 2012;337(6101):1471-1473. doi:10.1126/science.337.6101.1471

12. Chan AT, Cook NR. Are we ready to recommend aspirin for the prevention of cancer? Lancet. 2012;379(9826):1569-1571. doi:10.1016/s0140-6736(11)61654-1

13. Hampel H, Stephens JA, Pukkala E, et al. Cancer risk in hereditary nonpolyposis colorectal cancer syndrome: later age of onset. Gastroenterology. Aug 2005;129(2):415-421. doi:10.1016/j.gastro.2005.05.011

14. Burn J, Bishop DT, Chapman PD, et al. A randomized placebo-controlled prevention trial of aspirin and/or resistant starch in young people with familial adenomatous polyposis. Cancer Prev Res (Phila). May 2011;4(5):655-665. doi:10.1158/1940-6207.capr-11-0106

15. Cuzick J, Otto F, Baron JA, et al. Aspirin and non-steroidal anti-inflammatory drugs for cancer prevention: an international consensus statement. Lancet Oncol. May 2009;10(5):501-507. doi:10.1016/s1470-2045(09)70035-x

16. Earnshaw SR, Scheiman J, Fendrick AM, McDade C, Pignone M. Cost-utility of aspirin and proton pump inhibitors for primary prevention. Arch Intern Med. Feb 14 2011;171(3):218-225. doi:10.1001/archinternmed.2010.525