The 4 000-year-old drug, commonly used to treat pain, prevents certain tumours from forming and spreading across the body, findings that are already changing health policies, reports the BBC.
Britain’s Nick James, in his mid-40s, first became concerned about his health after his mother died from cancer and his brother, and several other relatives, later developed bowel cancer. He opted to undergo genetic testing, and was found to be carrying a faulty gene which causes Lynch syndrome, a condition that significantly increases the risk of developing that type of cancer.
Help came from an unexpected place, however, when James became the first person to sign up for a clinical trial that set out to test whether a daily dose of aspirin could protect against developing cancer.
Depending on the type of gene mutation, 10%-80% of people with Lynch syndrome will get bowel cancer during their lifetime. But so far, things are looking good for James. “He’s been on aspirin now with us for 10 years without any cancer so far,” said John Burn, a Professor of Clinical Genetics at Newcastle University, who led the trial.
There have long been indications that the drug might reduce the chances of colorectal cancer spreading, or even occurring in the first place, and in the past year, a string of trials and studies have strengthened such evidence.
Some countries have already changed their medical guidelines to include the pill as a first line of protection for those most at risk (though experts stress that this should only be done under a doctor’s supervision). And we’re finally beginning to understand the reasons why it has such a mysterious effect.
Ancient roots
The latest findings offer a remarkable new twist in the tale of one of our oldest and most effective medications. In the late 19th century, archaeologists uncovered 4 400-year-old clay tablets from the ancient Mesopotamian city of Nippur – now Iraq – offering lists of a range of medicines crafted from botanical, animal and mineral compounds. Among them were instructions for a substance derived from the willow tree.
We now know this contains salicin, which the body can convert into salicylic acid that helps to calm pain. It is very similar in structure to modern aspirin – acetylsalicylic acid – but more irritating for the stomach. Other ancient civilisations, including the Egyptians, Greeks and Romans, also used the remedy.
The modern study of the compound kicked off in 1763, when English cleric Edward Stone wrote to the Royal Society to describe the fever-fighting properties of dried and powdered willow bark. About a century later, scientists managed to synthesise salicylic acid into the less corrosive acetylsalicylic acid, and put it on the market under the brand name Bayer.
Fast forward yet another century, and scientists started noticing some unexpected benefits of aspirin on preventing cardiovascular disease – reducing the risk of blood clots by making the blood thinner and the blood platelets less sticky. For this reason, low daily doses are often recommended for people with a high risk of heart attack or stroke.
By 1972, the potential benefits had extended to the prevention of cancer, with an attention-grabbing study of mice injected with tumorous cells. The American scientists found that lacing the animals' drinking water with aspirin significantly reduced the risk that the cancer would spread across the body (metastasis) compared with mice who were not given the drug.
While the discovery generated some excitement, “it wasn’t immediately clear how this would affect clinical practice”, said Ruth Langley, a Professor of Oncology and medical trials at University College London. It wasn’t obvious if the drug would have the same effect in humans, after all, meaning that the finding remained an obscure fascination rather than a potentially life-changing treatment.
A turning point came in 2010, when Peter Rothwell, a Professor of Clinical Neurology at the University of Oxford, went back and re-investigated the much more abundant data on aspirin as a prevention of cardiovascular disease. In his analyses, the drug appeared to reduce both the incidence and spread of cancer, prompting renewed interest in both the power of aspirin to help fight the disease, and the reasons that it does so.
Proving that aspirin can prevent cancer in the general population is a challenge, however. In an ideal world, researchers would recruit a large sample of people. Half would take aspirin, while the rest would take a placebo, and you would then compare which had the highest rates of the disease.
But it can take many decades for cancer to occur in the first place, meaning that a randomised controlled trial would take a long time to conduct at a huge expense. “It’s almost impossible, actually,” said Anna Martling, a Professor of Surgery at the Karolinska Institute in Sweden.
For this reason, scientists have turned their attention to specific groups, like those who have already had cancer or who are genetically susceptible to developing it.
Mounting evidence
It is here that John Burn’s study of patients with Lynch syndrome, which vastly increases the risk of colorectal and other forms of cancer, enters the picture. In 2020, Burn published the results of a landmark randomised controlled trial of 861 patients with the condition.
Following the participants for 10 years, his team discovered that people who had taken a daily 600mg dose of aspirin for at least two years effectively halved their risk of colorectal cancer.
His team has since conducted a second trial, currently under peer review. The early results suggest a much lower dose of aspirin (75mg-100mg) is just as effective, if not more.
“Those who took aspirin for two years had 50% fewer cancers in the colon,” he said. “What we want to do is keep going for a few more years because the data will get better as time goes on.” (Nick James, the very first patient to enter the trial, was among those who appeared to have benefited.)
The low dose (75mg-100mg) is similar to what people take to prevent cardiovascular events. That matters, since aspirin can come with unpleasant side effects, including indigestion, internal bleeding, stomach ulcers and even brain haemorrhage, and lower dose can be much better tolerated.
The findings are already affecting policy. “In the UK, guidelines have been changed as a result of our findings,” said Burn. Since 2020, these now recommend that people with Lynch syndrome start taking aspirin at about 20 years old for most people, or 35 for less severe cases.
Given these results, it is natural to wonder whether aspirin could benefit other patient groups. Martling has investigated whether aspirin can reduce the risk of metastasis in people who’ve already had a diagnosis of colorectal cancer. Her team focused on people with common mutations in their bowel or rectal tumours.
“Of all patients getting colorectal cancer, 40% have one of the mutations we have studied,’ she said. Previous research had suggested these people may respond particularly well to aspirin.
The three-year-randomised controlled trial involved 2 980 patients, with one group taking 160mg of aspirin daily, starting within three months of surgery, and the other receiving a placebo. The aspirin-treated group had less than half the risk of recurrence – a highly significant effect size. “That’s a large group of the patients,” said Martling.
What’s more, both Martling’s and Burn’s trials showed very few cases of adverse effects in those taking aspirin.
Martling’s study, published in September 2025, quickly changed practice in Sweden. Since January 2026, bowel cancer patients in the country have started being screened for the mutations in question, and offered a low dose of aspirin if they have them.
It is not yet clear whether aspirin could protect patients from other cancers as well, but we may soon have some answers. Langley is currently running a large randomised controlled trial with 11 000 participants who've had colorectal, breast, gastro-oesophageal, or prostate cancer in the UK, Ireland and India.
Her team will be looking at the effect of a daily 100mg or 300mg preventative dose of aspirin, and they’re hoping to have results next year.
“We really are the first to explore the role of aspirin in other tumour types,” she said. She is aiming to replicate Martling’s findings for colorectal cancer, as well as gathering funds to investigate the implications of the specific mutations in the other cancers, too. The replication is key, she added, as authorities ideally want two sets of trial results before they make recommendations for patients.
How does it work?
The precise mechanism by which aspirin prevents cancer has long remained a mystery. “This fantastic drug works both within the cell and outside the cell,” explained Martling, so there could be several different mechanisms involved.
Her own works implicates an enzyme within the cell called Cox-2, which is inhibited by aspirin. This enzyme helps produce hormone-like compounds called prostaglandins, she said, which in turn activates a signalling pathway that can lead to uncontrolled cell growth.
Recent research by Rahul Roychoudhuri, a Professor of Cancer Immunology at the University of Cambridge, and his colleagues, suggests there may be another mechanism involving a gene that inhibits T-cells in the immune system from spotting and killing metastatic cancer cells.
They found that this gene can be activated by a clotting factor called thromboxane A2, which, as the name suggests, helps the blood to form clots after injury.
Since aspirin inhibits thromboxane, it may therefore render cancerous cells more visible to the immune system. This surprised the team.
Roychoudhuri’s research was conducted on mice, so we can’t be sure whether the results would also hold for humans. But intriguing research by Langley and her colleagues has shown that people who have had colorectal cancer or gastro-oesophageal cancer have much higher levels of thromboxane than healthy individuals, even up to six months after successful treatment, suggesting it may be a driver of metastases in humans, too.
A cure all?
Exactly who should be taking aspirin regularly, and when, remains a matter of debate. Some researchers believe the combined benefits for CVD and cancer should inspire wider uptake. Burn, who has taken aspirin as a preventative measure in the past, is optimistic about its potential for public health.
“We did a big study where we showed that if every 50-something year-old took a baby aspirin for ten years, the national mortality from all causes would be reduced by 4%,” Burn said.
Most researchers argue it should only be restricted to particular patients, however. “It’s one thing to give aspirin to a cancer population but it’s a totally different thing to offer the healthy population something that might harm them as well,” said Martling. That’s because aspirin can have serious adverse effects, and it isn’t likely to work for all people or all cancers.
See more from MedicalBrief archives:
Surprising link between aspirin, cancer deaths – ASPREE trial follow-up
Aspirin reduces cardiovascular events in elderly with raised Lp(a) – ASPREE analysis
Aspirin may lower colorectal cancer death risk – Italian study