Can aspirin help protect against colorectal cancers?

Aspirin is well known for its ability to ease pain from muscle aches and headaches; it reduces fevers; and low doses can thin blood, reducing the chance of clots that cause strokes and heart attacks. Now a new study suggests it may also play a role in colorectal cancer prevention.

Colorectal cancer, a cancer of either the large intestine or rectum, is the third most common type of cancer, and the second most common cause of death from cancer, worldwide. There were 1.9 million new cases diagnosed across the globe in 2020, according to the World Health Organization, and these numbers are expected to grow. In the United States, the rates of colorectal cancers have been rising in people younger than age 50 since the 1990s, which includes more young people dying from the disease, according to the National Cancer Institute. 

Now a new study published in the journal Cancer shows that colorectal cancer patients who took a daily dose of aspirin had a lower rate of metastasis to the lymph nodes and stronger immune response to their tumors. The research suggests that aspirin may be boosting the ability of the immune system to hunt for cancer cells.  

“It is a rather unexpected effect, because aspirin is mainly used as an anti-inflammatory drug,” says Marco Scarpa, a researcher at the University of Padova, and one of the authors of the study. As Scarpa notes, this study suggests that aspirin may be playing a slightly different role by stimulating the immune system’s surveillance response, which can then prevent or delay the progression of colorectal cancer. 

Your immune system is always surveilling the body for cells that just aren’t right. When they find such cancer cells, they will kill them just as they would kill invading bacteria or viruses, says Cindy Kin, a surgeon at Stanford University, who specializes in colon and rectal surgery. 

Colored 3D computed tomography scan of the colon of a 54-year-old male patient, showing extensive narrowing of the colon (highlighted, upper right). This suggests the presence of a cancerous lesion. Globally, colorectal cancer is the third most common form of cancer. 

Photograph by ZEPHYR/SCIENCE PHOTO LIBRARY

Confocal fluorescence light micrograph of colon cancer cells grown in the lab. The DNA-containing nuclei are colored blue; the tubulin of the cytoskeleton appears green.

MICROSCOPY AUSTRALIA, UNIVERSITY OF SYDNEY/SCIENCE PHOTO LIBRARY

“The data about aspirin and cancer is really evolving,” says Maen Abdelrahim, an oncologist at Houston Methodist Hospital, who specializes in treating colorectal cancers. However, there are still a lot of unanswered questions about how aspirin can prevent and delay the progression of these cancers, as well as which subset of patients would benefit from a daily aspirin.  

(Colon cancer is rising among young adults. Here are signs to watch for.)

People who take a consistent use of aspirin have a lower risk of colorectal cancer, “but it has to be balanced with the risks,” which includes the possibility of bleeding in the gastrointestinal tract, says Jeff Meyerhardt, an oncologist and co-director of the Colon and Rectal Cancer Center at the Dana-Farber Cancer Institute, in Boston.  

Aspirin protects against colorectal cancer

There are several studies that suggest a link between aspirin and colorectal cancer prevention and delay. However, the mechanism by which aspirin does this is still unknown. That makes it hard to predict which patients will benefit the most. 

In a 2020 meta-analysis, which analyzed the results of 45 observational studies, researchers found that regular aspirin use was associated with less incidence of colorectal cancer. 

A low dose, between 75 and 100 milligrams, was associated with a 10 percent reduction in the risk of developing colorectal cancer; a regular dose of 325 milligrams was associated with a 35 percent decline.  

Other studies have shown a link between daily aspirin and a delayed progression, including a lower risk of dying in patients who had already been diagnosed with colorectal cancer. 

“What has been seen in multiple studies for colorectal cancers is that having a more robust immune reaction does seem to have a better outcome,” Meyerhardt says. “This is looking at how aspirin may interact with that.” 

Study suggests mechanism

In the study, researchers obtained tissue samples from 238 patients who had undergone surgery for the colorectal cancers. Of these patients, 12 percent were taking a daily low dose of aspirin for the prevention of heart disease. When compared to patients who were not taking aspirin, the researchers found a lower rate of metastasis to the lymph nodes, and higher numbers of immune cells that had infiltrated the tumors. 

This higher level of infiltration is thought to be linked to slower cancer progression—including the lower rate of spread to the lymph nodes—by allowing immune cells to enter the tumor mass and fight the cancerous cells more effectively. 

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The researchers also found higher levels of immune markers that are responsible for triggering the immune system surveillance response. “It’s boosting the immune system, and it’s helping the immune system inside the tumor,” Abdelrahim says.  

In recent years, the immune system’s role in protecting against the development of cancer has become recognized. 

Patients with suppressed immune systems are at higher risk for developing cancers, compared to patients with a fully functional immune system. As these results suggest, aspirin may increase the vigilance of the immune system when it comes to the detection of colorectal cancers.  

“Your immune system is doing all of these things in the background, that you’re not even aware of,” Kin says. “It’s not just the tumor’s behavior and how aggressive it’s going to be, but it’s your body versus the tumor.”  

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