"Our most significant finding was that women who filled 70% or fewer of their tamoxifen prescriptions had a significantly -- 16% -- increased risk of death [compared with women who filled all their prescriptions]," says Alastair Thompson, MD, professor of surgical oncology at the University of Dundee in Scotland and one of the researchers.
In the study, one in 10 women filled 70% or fewer of their tamoxifen prescriptions, he tells WebMD.
Refills are a well-accepted surrogate for how many pills a person actually takes, as it is rare for people to buy medication if they still have some left.
Findings Apply to Newer Hormone Drugs, Too
While tamoxifen deprives breast cancer cells of the estrogen they need to grow, aromatase inhibitors actually block an enzyme needed to make estrogen, thereby slashing the body's production of estrogen altogether.
Nonetheless, tamoxifen still has an important role in breast cancer treatment, says Julie R. Gralow, MD, an associate professor of medical oncology at the University of Washington School of Medicine in Seattle, and moderator of a press briefing on the findings.
Aromatase inhibitors can only be given to postmenopausal women, leaving tamoxifen as the cornerstone of hormone therapy in younger women, she points out.
Plus, the less expensive tamoxifen is still more commonly prescribed in many other parts of the world, Gralow says.
The new findings also have implications for women taking aromatase inhibitors, she says.
"The study clearly shows that you need to take the majority of your pills to really get benefit. As we move toward greater use of aromatase inhibitors, which do not stay in the body as long, taking the drug as prescribed becomes even more important," Gralow tells WebMD.
Less Than Half of Women Took Tamoxifen for 5 Years
For the study, the researchers reviewed the records of 2,080 women treated for breast cancer between 1993 and 2002 in Scotland. Of the total, 79% were prescribed tamoxifen to reduce the risk of breast cancer returning after surgery.
Thompson notes that in Scotland, every patient has a unique 10-digit health number. This allowed the researchers to link data on each person's health status to pharmacy records.
Among the results:
- Although a 5-year course of tamoxifen is typically recommended, women took the drug for an average of 2.42 years.
- 37% of women stopped taking tamoxifen before 5 years, with 19% discontinuing its use 2 years into therapy.
- Less than half of women, 49%, took tamoxifen for 5 years.
- Women who didn't take tamoxifen at all had a 50% increased risk of dying, compared with women who took the drug.
Side Effects May Cause Fall Off
While the research was performed in Europe, U.S. studies have also shown that people with cancer often fail to take their drugs as prescribed, Thompson says.
Gralow advises women to talk about the issue with their doctors. "If you want to stop taking a drug because you don't feel good, it's really important to tell your doctor. There is often something else we can do for you," she says.
More than 180,000 new cases of breast cancer will be diagnosed in the United States in 2007, and more than 40,000 women will die from the disease, according to the American Cancer Society.