Postmenopausal women in the study who took aspirin, other nonsteroidal anti-inflammatory drugs (NSAIDs) such as Advil, Aleve, and Motrin, or Tylenol (acetaminophen) on a regular basis had lower estrogen levels than women who did not take the pain relievers.
The declines were modest, but the findings bolster suspicions that the painkillers may reduce the risk of most breast and ovarian cancers by suppressing the hormones that fuel them.
Study researcher Margaret A. Gates, ScD, says the association must be confirmed before analgesics can be recommended for cancer prevention. That's because the risks may outweigh potential benefits.
Gates is a research fellow in epidemiology at Boston's Brigham and Women's Hospital and Harvard Medical School.
"A randomized trial that directly measures the impact of analgesic use on hormone levels in a similar population of postmenopausal women would be helpful," she tells WebMD.
Earlier Studies Show Lower Cancer Risk
Over the last decade, no fewer than a dozen studies have suggested an association between frequent aspirin or NSAID use and a reduced risk for breast and ovarian cancer, but almost all the research has been observational.
In one of the most widely reported studies, Columbia University researchers questioned close to 3,000 women with and without breast cancer about their aspirin use.
They found a 20% lower breast cancer risk among women who said they were regular aspirin users, compared to infrequent aspirin users.
Just last year, Brigham and Women's researchers reported that breast cancer survivors who took aspirin regularly had a lower risk of cancer recurrence or death from their disease than women who did not take aspirin; the breast cancer survivors also had a lower risk of having their cancer spread beyond the breast.
The researchers followed 4,000 female nurses enrolled in the ongoing Nurses Health Study (NHS) who had been treated for breast cancer at least a year earlier.
Painkillers and Estrogen Levels
The newly published study included 740 postmenopausal NHS participants.
Researchers collected information on their use of analgesics between 1988 and 1990, and they also took blood samples from the women during this time.
Women who reported using the over-the-counter painkillers at least 15 days a month had estrogen levels that were 13% to 15% lower than women who reported no analgesic use, Gates says.
The finding suggests, but does not prove, a direct link between regular analgesic use and lower estrogen levels, American Cancer Society Vice President of Epidemiology and Surveillance Research Michael J. Thun, MD, says in a news release.
Thun serves on the editorial board of the journal Cancer Epidemiology, Biomarkers & Prevention, which published the study.
Like Gates, he called for new studies -- with participants randomly assigned to the use of painkillers -- to determine if analgesic use really does lower estrogen levels.
"Until then, we have a possible mechanism for a potentially important, but as yet unproven chemopreventive benefit," he notes.