An article that poses the alluring question “Cancer Treatment in Your Medicine Cabinet?” by Michelle Holmes and Wendy Chen, both Harvard physicians, has been getting a lot of traction. That’s understandable because of their premise that the common drug aspirin may be effective in reducing the death rate from breast cancer. Several observational studies, including one of their own, document that women who take aspirin for various reasons are about 50% less likely to die of breast cancer. Observational studies are notoriously difficult to interpret because other factors may be involved. For example, are women taking aspirin less likely to die of breast cancer because they succumb from some other condition for which they were taking aspirin? Low dose aspirin is commonly taken by people who are at increased risk for heart disease because of family history, prior heart attacks or atrial fibrillation. So are women who take aspirin dying from heart disease before breast cancer exerts its fatal effect, or perhaps of an aspirin side effect such as a stroke or gastrointestinal bleed? Aspirin is not a benign drug. The usual calculation is that there are about 3 major gastrointestinal bleeds and one hemorrhagic stroke for every 1000 individuals taking low dose aspirin over a five year period.
Of course it is possible that aspirin really does have an effect on breast cancer. After all, there is very good evidence that it does offer protection against colon cancer and less robust evidence for prevention of esophageal, stomach, prostate and ovarian cancer. Unfortunately, none of the studies that suggest protection are randomized trials, the most reliable type of study in which people are randomly assigned to take either aspirin or a placebo. Currently aspirin is not being recommended for prophylaxis against these cancers in healthy people because the risk-benefit profile is unclear. Furthermore effects may be different depending on age. People under 50 are less likely to see benefits, and side effects are more likely in people over the age of 75. The story is different for people who have had colon cancer, have a family history or who have had a large number of precancerous polyps removed. In such cases the benefits likely outweigh the risks.
For breast cancer, supposing that there is an effect, nobody knows how long it takes for it to “kick in” and what the appropriate dosage is. This being said, there are some encouraging laboratory studies with cell cultures and with mice that show a slowing of the growth of cancer cells and shrinkage of tumors as well as stopping tumour cells from spreading to new sites. A theory has been advanced that aspirin interferes with the formation of stem cells that are believed to fuel the growth and spread of tumors. Indeed, experiments with mice have shown that cancer cells treated with aspirin are less likely to form stem cells. But it is also possible that aspirin’s anti-inflammatory effect or anti-clotting effect is responsible. Some evidence indicates that drugs that interfere with platelet function prevent breast cancer from spreading in mice. Basically at this point there just isn’t enough evidence to determine if and to what extent aspirin may reduce cancer risk. But what we do know is that cancer risk can be cut by up to 50% by not smoking, drinking only in moderation, eating health foods and avoiding obesity.