Prolonged use of tamoxifen linked to high risk of breast cancer

The study established that continued use of the drug for five or more years reduced the common type of breast cancer by almost 60 percent, but increased the risk of a more aggressive subtype of the disease by more than 400 percent.

Not to be discontinued
Researchers concluded that the benefits of the drug still outweigh the negative effects; therefore tamoxifen should continue to be taken in its present form.

Researcher Christopher Li, MD, PhD, an associate member of the Fred Hutchinson Cancer Research Center said, “We don't think that it overall changes the risk-benefit equation, in that women who are eligible to take this drug probably should still take it because of its proven benefit.”

Tamoxifen prevents the growth of breast cancer cells by blocking the estrogen receptor. However, some breast cancers do not have estrogen receptor and these are the ones that are more belligerent and tend to spread more quickly.

Li said, "For the estrogen receptor-positive cancer, we have targeted therapy that again has been proven to again reduce mortality. That's one of the reasons why ER-negative cancers are more worrisome because we don't have a targeted treatment for them."

The study
For the purpose of the study, Li's team analyzed 1,100 Seattle-area women aged 40-79 who had been treated for estrogen receptor (ER-positive breast cancer between 1990 and 2005).

Out of the 1,100 patients, 367 women had reportedly developed breast cancer in the other breast after a minimum of six months after their first diagnosis.

The researchers not only interviewed these women but also checked their medical records and took note of the use of duration of tamoxifen or other hormone therapies.

The findings of the study revealed that Tamoxifen lowers breast cancer patients' risk of dying of the disease. However, the use of the drug enhances the risk of stroke and endometrial cancer, he added.

"I think the benefits of taking tamoxifen for someone who's already had a breast cancer and is trying to prevent recurrence of that breast cancer way outweigh the downside of what's shown in a study like this," said Pritchard, faculty of medicine at the University of Toronto.

The findings of the study have been published on-line in the journal 'Cancer Research'.