The latest on this issue comes from a National Cancer Institute study, which found that taking an aspirin every day does lower the risk - a bit - of the most common type of breast cancer. Some earlier research produced similar findings, but overall results have been inconsistent.
The new study, published April 30, 2008 in the open-access journal Breast Cancer Research, involved about 127,000 women between age 51 and 72, none of whom had cancer at the outset. Over the course of seven years, 4,501 of them developed breast cancer. Among women who took a daily aspirin - 18 percent of those participating - the risk of breast cancer was 16 percent lower than it was among those who didn't take aspirin or didn't take it every day. The lower risk applied only to estrogen-dependent breast cancer. The researchers also looked at daily use of ibuprofen but saw no effect from it.
The aspirin findings in the new study are similar to those from research published in 2004 showing that breast cancer risk was 28 percent lower in women who took aspirin seven or more times per week than in those who didn't use it regularly. In this earlier study as well, the protective effect applied only to estrogen-dependent breast cancer (the most common form of the disease).
In 2005, a study published in the Journal of the National Cancer Institute, contradicted the 2004 data. It found no effect from aspirin use, overall, although it did show a 50 percent increased risk among women who took ibuprofen for five years and an 80 percent increase in the risk of estrogen-receptor-negative breast cancer (a much less common type) among women who took aspirin daily.
Based on what we know now, I wouldn't recommend taking aspirin to reduce your risk of breast cancer. (I do recommend it for other reasons, especially for cardiovascular benefits.) We still don't know whether its protective benefit against estrogen-dependent breast cancer - if any - outweighs the known risks of long-term use of aspirin, especially stomach irritation and gastrointestinal bleeding.
Andrew Weil, M.D.