A major review published in the Cochrane Library, cited by HealthDay, shows that daily aspirin use may increase the risk of serious bleeding.
There are also indications of a possible increased risk of bleeding in the brain.
The risk applies even at low doses. According to the researchers, the risk of bleeding begins from the moment a person starts taking the medication.
Any potential protective effect against cancer, by contrast, would only become apparent after more than ten years, if it occurs at all.
Also read: Women face greater risk of dying after heart attack, study finds
What does the study show?
The analysis is based on 10 randomized trials involving nearly 125,000 participants.
The researchers examined whether aspirin could prevent colon and rectal cancer or precancerous conditions in individuals at average risk.
During the first five to fifteen years, there was little or no protective effect.
Some studies suggested a possible benefit after long-term use, but the researchers assess the certainty of this evidence as very low.
Also read: Early findings offer new hope in fight against pancreatic cancer
Caution recommended
Aspirin and similar painkillers have been studied for several years as a possible means of cancer prevention.
Interest has partly focused on their effect on inflammatory processes in the body.
The researchers behind the review, who are affiliated with West China Hospital at Sichuan University, recommend consulting a doctor before starting aspirin to prevent cancer.
Overall, the review shows that the benefits for individuals at average risk are uncertain, while the side effects are well known and can be serious.
Also read: More hours of sleep may lead to weight loss, study shows
Sources: HealthDay, and Cochrane Library.
Also read: Can plant-based diets provide enough protein? Experts have the answer