Although the studies reviewed for this report don't prove aspirin prevents cancer, they offer strong evidence that it might, Lichtenfeld said.
Still, taking aspirin has risks. "Some people will be at risk of stomach bleeding, but very few," Cuzick said.
People 60 years old who take daily aspirin for 10 years have an increased risk of stomach bleeding of about 3.6 percent. Bleeding could be life-threatening in less than 5 percent of people who develop bleeding, the researchers noted.
The risk of serious bleeding, however, increases dramatically after age 70. Cuzick recommends that people 70 and older not start taking aspirin to prevent cancer because of this increased risk.
Peptic ulcers are another side effect of aspirin. The studies the authors reviewed cited an increased risk of 30 to 60 percent for these stomach lesions.
In terms of benefits, it's still unknown what dose of aspirin provides the maximum protection, Cuzick said.
"The evidence suggests that low-dose aspirin (75 milligrams) is as effective as the standard dose of 300 milligrams, but there has been no direct comparison," he said. "So people should take the low dose, but research should be done to see if the standard dose is even more effective."
Cuzick said aspirin's protective effect doesn't appear to kick in until it's taken for at least five years, and probably 10 years, between the ages of 50 and 65. No benefit was seen in the first three years, and it's not clear if taking aspirin for more than a decade will reap greater benefits, he said.
Taking an aspirin daily should not be seen as a reason not to be screened for cancer, Lichtenfeld cautioned. "Screening has an important impact on reducing colon cancer," he said.
As to why aspirin is protective, Cuzick can only speculate. It's known that aspirin interferes with blood-clotting by reducing platelets in the blood. Platelets are thought to help cancer cells travel thr
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