SAN ANTONIO - Taking menopause hormones for five years doubles the risk for breast cancer, according to a new analysis of a big federal study that reveals the most dramatic evidence yet of the dangers of these still-popular pills.

Even women who took estrogen and progestin pills for as little as a two years had a greater chance of getting cancer. And when they stopped taking them, their odds quickly improved, returning to a normal risk level roughly two years after quitting.

Collectively, these new findings are likely to end any doubt that the risks outweigh the benefits for most women.

It is clear that breast cancer rates plunged in recent years mainly because millions of women quit hormone therapy and fewer newly menopausal women started on it, said the study's leader, Rowan Chlebowski of Harbor-UCLA Medical Center in Los Angeles.

"It's an excellent message for women: You can still diminish risk [by quitting], even if you've been on hormones for a long time," said Claudine Isaacs of Georgetown University's Lombardi Comprehensive Cancer Center. "It's not like smoking, where you have to wait 10 or 15 years for the risk to come down."

Study results were given yesterday at the San Antonio Breast Cancer Symposium.

They are from the Women's Health Initiative, which tested estrogen and progestin pills that doctors long believed would prevent heart disease, bone loss, and many other problems in women after menopause. The main part of the study was stopped in 2002 when researchers saw surprisingly higher risks of heart problems and breast cancer in hormone users.

Since then, experts have debated whether these risks apply to women who start on hormones when they enter menopause, usually in their 50s, and take them for shorter periods of time. Most of the women in the federal study were in their 60s and well past menopause.

So the advice has been to use hormones only if symptoms such as hot flashes are severe, and at the lowest dose and shortest time possible. The new study sharpens that message, Chlebowski said.

"It does change the balance" on whether to start on treatment at all, he said.

Even so, most women will not get breast cancer by taking the pills short-term. The increased cancer risk from a couple of years of hormone use translates to a few extra cases of breast cancer a year for every 1,000 women on hormones. This risk accumulates with each year of use, though.