A new UK study suggests that a daily dose of 300 mg aspirin reduces the occurrence of colorectal cancer by up to 74 per cent. However, doctors are not recommending it because of possible stomach problems such as bleeding, but it might be a worthwhile precaution for people at high risk of colorectal cancer.
The study is published in The Lancet.
Previous trials have shown that aspirin reduces the short-term risk of benign growths or adenomas in the bowel of patients with a history of adenomas or cancer. However, they failed to show that aspirin prevents colorectal cancer during ten years of follow up.
In this study the researchers looked at more than 10 years of follow up because the time it takes for early cancer to develop after the appearance of adenomas is at least that.
Also, previous studies were of the observational type, looking back at the use of aspirin by bowel cancer patients and healthy subjects rather than randomizing participants from the start to various doses and a placebo and then following their progress.
Professor Peter M Rothwell of the Radcliffe Infirmary, Oxford, UK and colleagues did a 10 to 14-year follow up of two large British trials that started in the 1970s and early 1980s that included more than 7,500 people.
Using national cancer registers, they followed the incidence of colorectal cancer in the trial participants for a median period of 23 years. The total number of colorectal cancer cases that arose in that period was 215.
They found that a daily dose of 300 mg or more of aspirin reduced the incidence of colorectal cancer by between 63 and 74 per cent.
One trial, called the British Doctors Aspirin Trial had enrolled over 5,000 male doctors who took either 500 mg of aspirin or no aspirin for between 5 and 6 years.
The other trial, called the Transient Ischemic Attack Aspirin Trial (UK-TIA) covered nearly 2,500 patients who had suffered a TIA or minor stroke. They took a daily dose of either 300 or 1,200 mg of aspirin or a placebo for between 1 and 7 years.
Prof Rothwell and colleagues also conducted a systematic review of all relevant observational studies to see if they came to the same conclusions and if so what might be said about dose and regularity of aspirin use, plus the use of other non-steroidal anti-inflammatory drugs (NSAID) and how patient characteristics might affect the situation.
Pooling the results from the trials the researchers found that:
- Allocation to aspirin reduced the occurrence of colorectal cancer.
- However, the effect only kicked in after 10 years.
- The effect also depended on duration of treatment and whether patients stuck to a daily dose.
- The effect was greatest 10 to 14 years after the start of treatment for patients who were on it for at least 5 years.
- There was no record of a significant effect on other cancers.
The review of the observational studies also showed that in 19 case controlled studies covering nearly 21,000 people, and 11 cohort studies covering over 1.1 million people, regular use of aspirin or NSAID, was linked consistenly with reduced risk of colorectal cancer, especially after 10 years or more of use.
Also, there was no difference between NSAIDs and aspirin in the observationals studies, and these results were independent of age, sex, race, family history, the site of the cancer or its aggressiveness. As with the ramdomized trials, daily doses below 300 mg of aspirin however, had less consistent effect, and they diminished and became less consistent as dosage got lower and less frequent.
The researchers concluded that:
“Use of 300 mg or more of aspirin a day for about 5 years is effective in primary prevention of colorectal cancer in randomised controlled trials, with a latency of about 10 years, which is consistent with findings from observational studies.”
They also said that long term follow up from other clinical trials is needed to establish the effects of taking a lower or less frequent dose of aspirin.
In an accompanying editorial, Dr Andrew T Chan, from the Massachusetts General Hospital in Boston, USA, said these results are not consistent with long term studies of low dose aspirin, including the Physicians’ Health Study and the Women’s Health Study.
Perhaps this is because it is only the higher dose that prevents or limits adenomas forming as a result inhibiting cyclo-oxygenase-2 (COX-2), the enzyme responsible for inflammation and pain.
However, Dr Chan suggests these results are not sufficient evidence to recommend the general population starts long term use of aspirin to prevent colorectal cancer, especially since other alternatives such as screening are available.
According to Cancer Research UK, around 100 new cases of colorectal cancer are diagnosed each day in the UK. It is the third most common cancer after breast and lung cancer. Over 16,000 people die from it every year.
In 2003 there were over 35,000 new cases of large bowel cancer in the UK, with about two thirds of them being colon cancer and one third rectal cancer.
“Effect of aspirin on long-term risk of colorectal cancer: consistent evidence from randomised and observational studies.”
Enrico Flossmann, and Peter M Rothwell.
The Lancet 2007; 369:1603-1613.
Volume 369, Number 9573, 12 May 2007.
DOI:10.1016/S0140-6736(07)60747-8.
Click here for more information about colorectal cancer from Cancer Research UK.
Written by: Catharine Paddock
Writer: Medical News Today