BOSTON -- Preventing breast cancer before it develops is not only possible, it's happening right now thanks to a new use for Tamoxifen, a common breast cancer treatment drug.

"We have a new tool with which we can strike down breast cancer," said Dr. Paul Goss, director of breast cancer research at Massachusetts General Hospital.

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"The women taking tamoxifen were half as likely to get breast cancer," said Dr. Judy Garber, director of the Center for Cancer Risk and Prevention at Dana-Farber Cancer Institute

All Patients had to do was take one pill daily for five years.

"It's like you're buying a lifelong insurance policy, but you pay the installments in five years," said Goss.

Right now, it's a treatment that's only for women considered very high risk. That includes women who have had breast cancer before, have the BRCA gene mutation, have a family history or have had a biopsy showing changes in their breast tissue. Those factors can raise a woman's risk of getting breast cancer to anywhere from 35 percent to 85 percent, according to the Mayo Clinic. The average woman without those risks has a 12 percent chance of getting it in her lifetime.

"To cut your breast cancer risk by at least 50 percent and have that benefit last another 15 years? ... We have hardly any drugs in cancer or heart disease that are that powerful," said Garber.

Breast cancer affects nearly 200,000 people every year, according to Internal Medicine News. But of thousands of eligible women, only 3 percent have chosen to take tamoxifen for prevention, said Garber. The main reason is concern over dangerous, possibly deadly side effects, including blood clots and cancer of the uterus.

"People are worried about those side effects, which makes sense," said Garber. "Now for the very youngest women, women under 50 with high risk, actually, the side effects are incredibly rare."

But for women older than 50, other drugs may offer similar benefit with less risk Raloxifene, also known by its brand name Evista, cuts the risk in post-menopausal women by half, according to one study. Exemestane, or Aromasin, lowers it by 65 percent.

"It's huge. It's absolutely massive," said Goss.

All three drugs have less dangerous side effects, including hot flashes, muscle cramps and other menopausal symptoms. Aromasin can also cause bone loss, so doctors don't recommend it for women already struggling with weak bones.

To find out your breast cancer risk, click here.