Regular aspirin use has been linked to abdominal bleeds.
Taking aspirin in your 40s could cut the risk of cancer later in life, a review of research suggests.
Experts said taking the drug at an age before cancer usually develops, and for ten years would maximise its potential to prevent the disease.
Aspirin has been linked to a reduced risk of some cancers, and heart disease, but also to a raised risk of ulcers and internal bleeding.
The Cancer Research UK study features in the journal Lancet Oncology.
Aspirin blocks the effects of proteins which can trigger inflammation, and which are found at unusually high levels in several types of cancer.
Previous research suggests people who take the drug are less likely to develop bowel, breast and possibly some other types of cancer.
However, regular use of aspirin specifically for cancer prevention is not currently recommended because of the risk of side effects.
Common cancers, such as prostate, breast, lung and bowel, tend to develop after the age of 60 - when the risk of aspirin causing internal bleeding is at its highest.
Lead researcher Professor Jack Cuzick, from the Cancer Research UK Centre for Epidemiology at Queen Mary, University of London, said pre-cancerous lesions tended to start developing in the mid-40s.
Thus, taking aspirin around that time may be the best strategy for preventing that damage progressing to the full-blown disease.
It would also carry a much lower risk of side effects than beginning to take aspirin 15-20 years later.
However, Professor Cuzick said: "Many questions need to be answered before we would advise regular use of aspirin for cancer prevention.
"Future research and more clinical trials are needed to better identify those people who are at high risk of developing cancers and at low risk of side effects, who will benefit most from aspirin treatment."
Professor Cuzick said it was not clear a lower dose "baby aspirin" could achieve the same anti-cancer effect as the standard dose of 300mg/day.
The researchers also found that taking aspirin in combination with other drugs known as proton pump inhibitors could help to lower the risk of stomach bleeding.
Dr Lesley Walker, Cancer Research UK's director of cancer information, said: "It's too soon to recommend that people take aspirin to try and stop cancer developing because of the side effects.
"It's important that any decision to take aspirin regularly is only made in consultation with a GP."
Ellen Mason, of the British Heart Foundation (BHF), also stressed it was too early for researchers to recommend taking aspirin to reduce the risk of cancer.
"Currently the risk of bleeding outweighs the benefit," she said.
"Many thousands of people in the UK are prescribed aspirin because they have heart disease.
"This research does not prove that they will also get protection from cancer at a low dose, but as they already need to take aspirin it would be reassuring if further research eventually shows an anti-cancer benefit."