New evidence suggests that aspirin might help ward off four types of cancer, but doctors say don't start popping the pills yet.
A study released Sunday by the American Cancer Society found that deaths from stomach, esophagus, colon and rectal cancers were lower among people who used aspirin every other day.
Earlier research by the cancer society indicated that aspirin lowered the risk of colon cancer alone.
Dr. Michael Thun, who conducted both studies on the same 635,000 Americans, said the new findings indicate aspirin may inhibit tumor growth or boost the body's natural cancer-fighting cells.
"But these findings need to be confirmed," he cautioned. "People shouldn't just start taking aspirin because of its potential toxicity. It causes ulcers and serious side effects in some people."
When Thun published his first study on aspirin and colon cancer risk in 1991, doctors touted it as substantial evidence to prompt clinical trials of the possible link. Those trials are under way.
The new evidence is a little more puzzling, said Dr. Peter Greenwald, director of cancer prevention at the National Cancer Institute in Bethesda, Md.
"It would be unusual for one thing to be protectant against all four cancers, but it's not impossible," he said. "The gold standard of evidence in cancer research are clinical trials. This is very interesting suggestive evidence, but we really need those clinical trials."
Aspirin already is touted as a weapon against heart attack because it helps prevent blood clots.
Studies on animals and small groups of people also suggested it lowered the risk of colon cancer, so Thun studied 635,000 people between 1982 and 1988.
Those who took aspirin at least 16 times a month had a 40% lower risk of dying of colon cancer than non-users, he reported in 1991.
For the new study, published in the journal Cancer Research, Thun looked at deaths from any type of cancer in those same people. He did the follow-up because new animal experiments suggested aspirin could affect numerous cancers, even breast and lung cancers.