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This 4,000-year-old medicine, mostly used to treat pain, prevents some tumors from forming and spreading throughout the body, findings that are already changing health policy.
Nick James, a British furniture maker in his mid-40s, began to worry about his health after his mother died of cancer, and his brother and several other family members developed bowel cancer.
James chose to undergo genetic testing, and it was found that he carried a “defective” gene that causes Lynch Syndrome, a disease that greatly increases the risk of developing this type of cancer.
But help came from an unexpected place, when James became the first person to enroll in a clinical trial aimed at testing whether a daily dose of aspirin, an over-the-counter painkiller, could protect against cancer.
About 80 percent of people with Lynch syndrome will develop bowel cancer during their lives. But so far, things are looking good for James.
Depending on the type of genetic mutation, the incidence of bowel cancer ranges from 10 to 80 percent among people with Lynch syndrome during their lifetime.
But so far, James is doing well. “James has been taking aspirin with us for 10 years and has not developed cancer yet,” says John Byrne, professor of clinical genetics at Newcastle University, who led the trial.
It seems almost impossible to believe, yet there have long been indications that this drug may reduce the chances of colorectal cancer spreading, or even occurring in the first place.
Over the past year, a series of experiments and studies have strengthened this evidence.
Some countries have already amended their medical guidelines to include birth control pills as a first line of defense for people at high risk of infection (although experts stress that this should only be done under the supervision of a doctor).
Finally, we are beginning to understand the reasons why it has this mysterious effect.
Old roots
The latest discoveries show an exciting new twist in the story of one of the oldest and most effective medicines. In the late nineteenth century, archaeologists discovered 4,400-year-old clay tablets from the ancient city of Nippur in Mesopotamia (now Iraq) containing lists of a group of medicines made from plant, animal and mineral compounds.
Among them are instructions for a substance extracted from the willow tree, which we now know contains a chemical called salicin, which the body can convert into salicylic acid, which helps relieve pain. It is similar in composition to modern aspirin, acetylsalicylic acid, but is more irritating to the stomach.
Other ancient civilizations, including the Egyptians, Greeks, and Romans, also used this treatment.
Modern study of this compound began in 1763, when the English clergyman, Edward Stone, wrote to the Royal Society to describe the fever-fighting properties of dried and ground willow bark.
About a century later, scientists synthesized salicylic acid into the less corrosive acetylsalicylic acid and put it on the market under a brand name called Bayer.
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Another century later, scientists began to notice some unexpected benefits of aspirin in preventing cardiovascular disease, by reducing the risk of blood clots by making the blood more fluid and the platelets less sticky.
For this reason, organizations such as the UK’s National Health Service recommend low daily doses for people at high risk of having a heart attack or stroke.
By 1972, the potential benefits had expanded to include cancer prevention, through a remarkable study conducted on mice injected with cancer cells. American scientists found that adding aspirin to the animal’s drinking water significantly reduced the risk of cancer spreading throughout the body – or what is known as metastasis – compared to mice that were not given the drug.
Although the discovery sparked some enthusiasm, Ruth Langley, professor of oncology and clinical trials at University College London, said that “it was not immediately clear how this would affect clinical trials.”
It was also not clear, after all, whether the drug would have the same effect on humans, meaning the discovery remained a mystical magic rather than a potentially life-changing treatment.
The year 2010 marked a turning point, when Peter Rothwell, a professor of clinical neurology at the University of Oxford in the United Kingdom, re-examined the abundant data on aspirin as a preventive measure for cardiovascular disease. His analyzes showed that the drug reduces both the incidence and spread of cancer, sparking renewed interest in both aspirin’s ability to help fight the disease, and the reasons why.
However, proving that aspirin can prevent cancer in the general population is challenging. In an ideal world, researchers would recruit a large sample of people, half of them taking aspirin, the other half taking a placebo, and then compare the two groups to determine which has the highest rates of disease.
It can take many decades for cancer to develop, which means that conducting a randomized controlled trial would take a very long time and be expensive. “It’s actually almost impossible,” explains Anna Martling, a professor of surgery at Karolinska Institutet in Sweden.
For this reason, scientists have turned their attention to specific groups, such as those who have already had cancer or those who are genetically susceptible to it.
Growing evidence
Here the importance of John Byrne’s study for patients with Lynch syndrome, which greatly increases the risk of colorectal cancer and other types of cancer, is highlighted. In 2020, Byrne published the results of a groundbreaking, randomized, controlled clinical trial that included 861 patients with this syndrome.
After following the participants for 10 years, his team discovered that people who took a daily dose of 600 milligrams of aspirin for at least two years had their risk of developing colorectal cancer reduced by about half.
His team has since conducted a second experiment, which is currently undergoing peer review. Preliminary results suggest that a much lower dose of aspirin, between 75 and 100 milligrams, is just as effective, if not more so.
“The incidence of colon cancer in people who took aspirin for two years decreased by 50 percent,” says John Byrne.
“We want to continue the experiment for a few more years because the data will improve over time,” he adds. Nick James, the first patient to join the trial, was among those who appeared to benefit from it.
The low dose was 75 to 100 milligrams, a dose similar to what people take to prevent cardiovascular disease. This is important, because aspirin may cause unwanted side effects, including indigestion, internal bleeding, stomach ulcers, and even brain bleeding, and a lower dose is much better tolerated. These findings have already influenced health policies.
“In the UK, guidelines have been changed due to our findings,” Byrne says.
Since 2020, these guidelines recommend that people with Lynch syndrome start taking aspirin at approximately age 20 for most people, or at age 35 for less severe cases.
Given these findings, it is natural to wonder whether aspirin might benefit other groups of patients. Martling has investigated whether aspirin reduces the risk of cancer spreading in people diagnosed with colorectal cancer.
Her team focused on people who have common mutations in bowel or rectal tumors. “Among all colorectal cancer patients, 40 percent have one of the mutations we studied,” she explains. Previous research has indicated that these people may respond well to aspirin.
The three-year, randomized controlled trial included 2,980 patients, with one group taking 160 milligrams of aspirin daily, starting three months after surgery, while the other group received a placebo. The risk of disease recurrence in the group treated with aspirin decreased to less than half, which reflects the size and significance of this effect.
“This is a large group of patients,” says Martling. Furthermore, both trials, the Martling trial and the Byrne trial, showed very few cases of side effects in people who took aspirin.
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Martling’s study, published in September 2025, led to a rapid change in medical practices in Sweden. Since January 2026, bowel cancer patients in the country have been screened to detect the aforementioned genetic mutations, and given low-dose aspirin if they are present.
It is not yet clear whether aspirin protects patients with other types of cancer as well, but we may soon get some answers.
Langley is currently running a large, randomized clinical trial involving 11,000 participants with colorectal, breast, esophageal or prostate cancer in the UK, Ireland and India. Her team will investigate the effect of a daily preventive dose of 100 milligrams or 300 milligrams of aspirin, and they hope to have results next year.
“We are really the first to explore the role of aspirin in other types of tumors,” Langley says.
She seeks to replicate Martling’s findings in colorectal cancer, in addition to raising the necessary funding to study the effects of specific mutations in other types of cancer as well. She says that replication is crucial, as health authorities prefer to obtain the results of two sets of clinical trials before making any recommendations to patients.
How do experiments work?
The exact mechanism by which aspirin protects against cancer has long been a mystery.
“This amazing drug works inside and outside the cell,” Martling explains, so there may be several different mechanisms involved. Her research suggests there is an enzyme inside the cell called COX-2, which we know is inhibited by aspirin. This enzyme helps produce hormone-like compounds called prostaglandins, which in turn activate a signaling pathway that may lead to uncontrolled cell growth, she says.
Recent research by Rahul Roychoudhury, a professor of cancer immunology at the University of Cambridge, UK, and colleagues suggests that another mechanism may involve a gene that prevents T cells (a type of white blood cell) in the immune system from detecting and killing metastatic cancer cells (malignant cells that break off from the original tumor and travel through the blood or lymph system to form new tumors).
The researchers found that this gene can be activated by a clotting factor called thromboxane A2, which, as its name suggests, helps blood form clots when injured. Since aspirin inhibits thromboxane, it may make cancer cells more visible to the immune system, which came as a surprise to the team.
Roychowdhury’s research was conducted on mice, so we cannot be certain that the results will also apply to humans. But interesting research by Langley and her colleagues showed that people who developed colorectal cancer or esophageal and stomach cancer had significantly higher levels of thromboxane than healthy individuals, even six months after successful treatment, suggesting that it may be a catalyst for cancer spread in humans as well.
A cure for all diseases?
Controversy still exists about who should take aspirin regularly, and when.
Some researchers believe that its combined benefits in preventing cardiovascular disease and cancer should encourage its wider use.
Webern, who has taken aspirin as a preventive measure in the past, is optimistic about its public health potential.
“We did a large-scale study that showed that if every person in their 50s took low-dose aspirin for ten years, the national death rate from all causes would decrease by four percent,” Byrne says.
Despite this, most researchers believe that the use of aspirin should be limited to specific patients only.
“It is possible to give aspirin to cancer patients, but that is completely different from giving healthy people a drug that may also harm them,” Martling says. This is because aspirin can cause serious side effects and is unlikely to be effective for all people or all types of cancer.
If you have Lynch syndrome or have undergone treatment for bowel cancer, it may be worth asking whether a regular low dose might be helpful.
“Always talk to your doctor or other health care professional before starting aspirin,” Langley says.
As research on aspirin continues, the coming days may bring us surprises. But will aspirin’s long history extend another 4,000 years into the future? Perhaps our descendants will use versions of this medicine in ways we can’t even imagine.
