TUESDAY, March 23, 2010 (HealthDay News) -- Postmenopausal women who regularly take aspirin or other painkillers have lower estrogen levels than nonusers, a new study shows, which might explain a decreased risk of breast or ovarian cancer among these women.
"There is a lot of research on analgesic use and breast cancer risk," said Margaret Gates, a research fellow at the Channing Laboratory at Brigham and Women's Hospital and Harvard Medical School, Boston. "We didn't look at the association with breast cancer," she said. "We looked at analgesics and levels of estrogen as a possible mechanism for analgesic use and breast cancer [risk reduction]."
In the study, Gates and her team evaluated painkiller habits -- use of aspirin, non-steroidal anti-inflammatory drugs and acetaminophen -- among 740 postmenopausal women who participated in the Nurses' Health Study. All women reported their painkiller use in 1988 or 1990, then provided a blood sample in 1989 or 1990.
"Levels of estrogen [in the blood] were lower among women who used analgesics 15 or more days per month compared to women with no analgesic use," Gates said.
The reduction was 12 percent to 15 percent, depending on which estrogen form was evaluated, she said. The findings appear in the April issue of Cancer Epidemiology, Biomarkers & Prevention.
Exactly why painkillers lower estrogen levels is not certain. But Gates said one hypothesis is that the painkillers inhibit the expression of an enzyme, aromatase, that converts testosterone to a form of estrogen.
"If you block that, you're not making as much estrogen," she said.
The finding may explain the association between painkiller use and lower breast cancer risk, she said. "The data is very strong linking lower estrogen levels to lower risk of breast cancer," Gates said. Data for a link between ovarian cancer is also there, but less strong," she said.
The new data is "incredibly strong," said Mary Beth Terry, an associate professor of epidemiology at Columbia University's Mailman School of Public Health in New York City, who has found in her research that regular aspirin users are at lower risk of breast cancer than those who don't take painkillers.
The link she found is for hormone-receptor positive breast cancer, the most common type. Such cancers grow when exposed to estrogen.
The new study is the largest, Terry said, to look at aspirin and other analgesic use and measure blood levels of estrogen.
Gates and Terry stopped short of recommending women take analgesics to reduce breast cancer risk. "I think more research is needed before making recommendations," Gates said. Some women are already taking the drugs to relieve arthritis or for heart protection, she said.
Despite the benefits found for painkillers, there can be downsides, such as gastrointestinal bleeding, experts note. Women should ask their doctor what's best for them when it comes to painkiller use, Gates said.
To learn more about aspirin and breast cancer risk, visit the American Cancer Society.