- Colorectal, or bowel, cancer is the third most common cancer worldwide, making up 10% of cancer diagnoses.
- It most commonly affects people over the age of 50, but cases in younger people are increasing.
- Studies have found that in people with a genetic risk, such as Lynch syndrome, daily aspirin can cut their risk of colorectal cancer.
- However, a new review has found that, for the general population, daily aspirin appears to offer no protection against colorectal cancer, and may have other adverse effects.
Colorectal, or bowel, cancer is the third most common cancer worldwide. According to the World Health Organization (WHO), in 2022 alone there were 1.9 million new cases of colorectal cancer and it caused more than 900,000 deaths around the world.
Although most cases occur in people over the age of 50, in some countries incidence is rising in younger people. But the condition can be prevented by following a healthy lifestyle, and screening to detect early signs.
Some research has suggested that, in people with a genetic risk for colorectal cancer, a daily low dose of aspirin can reduce that risk.
Other studies have shown that it may help prevent recurrence in people who have had colorectal cancer.
But a new review of the evidence, carried out by researchers from West China Hospital of Sichuan University in China, and published in the Cochrane Database of Systematic Reviews, suggests that for people without genetic risk or prior colorectal cancer, there is no evidence that taking daily aspirin reduces colorectal cancer risk.
Lead author Bo Zhang, MD, of the Department of General Surgery, West China Hospital, Sichuan University in Chengdu, China, co-senior author of the review stated in a press release that:
“As scientists, we must follow the evidence where it leads. Our rigorous analysis of the highest-quality trials reveals that the ‘aspirin for cancer prevention’ story is more complex than a simple ‘yes or no.’ The current evidence does not support a blanket recommendation for aspirin use purely to prevent bowel cancer.”
The researchers included 10 trials in their analysis, with a total of almost 125,000 participants. All trials were comparing the effect of aspirin against placebo or no intervention on the risk of primary colorectal cancer or precancerous polyps (colorectal adenoma) in older adults.
Participants’ mean ages ranged from 53 to 71 years, and most were white. Seven of the studies used a low daily dose of 75-100 milligrams (mg) of aspirin, although three used higher doses of up to 500 mg per day.
All studies measured incidence of colorectal cancer within the follow up period that ranged from 5 to more than 15 years. Six studies also recorded deaths from colorectal cancer.
In addition, the studies recorded serious adverse events of aspirin treatment, including bleeding events such as extracranial hemorrhage (bleeding outside the skull but under the scalp) and hemorrhagic stroke (bleeding within the brain usually due to blood-vessel rupture).
From their analysis, the researchers came to the following conclusions:
- Aspirin probably has little or no effect on incidence of colorectal cancer or colorectal adenoma at 5-15 years of follow up.
- Aspirin may increase colorectal cancer mortality at 5-10 years of follow up. The researchers suggest this may be because aspirin promotes progression of pre-existing, undetected cancers.
- Aspirin may reduce colorectal cancer incidence and mortality after 15 years or more, but the researchers viewed this evidence as uncertain as at this time interval, participants may have stopped aspirin, started it independently, or begun other treatments.
They also advised that while aspirin had little effect on serious adverse events, it did increase the likelihood of serious extracranial hemorrhage, and probably increased the risk of hemorrhagic stroke.
These bleeding risks were greater with higher doses of aspirin.
“This review reinforces that we must move away from a one-size-fits-all approach,” Dan Cao, MD, of the Division of Abdominal Tumor Multimodality Treatment, Cancer Center, West China Hospital, Sichuan University, who is co-senior author of the review, said in a press release.
“Widespread aspirin use in the general population simply isn’t supported by the evidence,“ added Cao. “The future lies in precision prevention — using molecular markers and individual risk profiles to identify who might benefit most and who is most at risk.”
Low dose daily aspirin is recommended for people with cardiovascular disease, to reduce the risk of further cardiovascular events, such as heart attacks or strokes.
Aspirin decreases the activity of blood platelets — the benefit is that it reduces the likelihood of blood clots, the disadvantage is that it increases the risk of bleeding events.
For people without existing cardiovascular disease, as the risk of bleeding events outweighs the potential benefit of preventing cardiovascular events, daily aspirin is not advised.
These people would benefit more from maintaining a healthy lifestyle, and keeping their blood pressure and blood lipids at healthy levels.
The anticancer properties of aspirin have been debated. Some studies suggest aspirin may reduce the risk, particularly in those with previous adenomas; others, including this latest analysis, have found no anti-cancer benefit.
Speaking to Medical News Today, Cancer Research UK highlighted that aspirin may have uses in cancer treatment and prevention but further research is needed.
They are investigating whether aspirin might be a beneficial extra treatment for cancer, or help to prevent cancer recurring after treatment.
And this study highlights the bleeding risks of taking daily aspirin, particularly at higher doses. Zhang expressed some caution:
“My biggest worry is that people might assume that taking an aspirin today will protect them from cancer tomorrow. In reality, any potential preventive effect takes over a decade to appear, if it appears at all, while the bleeding risk begins immediately.”
