Over the past 30 years, millions of women have taken Tamoxifen after breast cancer.
It is a drug used to help cut the odds of the cancer coming back.
Most breast cancer patients take it for five years, but a new study shows taking it longer is even better.
Researchers followed nearly 7,000 breast cancer survivors, and found those who took Tamoxifen for 10 years were less likely to get a recurrence than women taking the drug for five years.
And younger women, those who hadn't reached menopause, had the biggest decrease.
The results were a surprise because doctors have long believed that more than 5 years of Tamoxifen didn't help.
The drug works by blocking the effect estrogen has on cancer cells. Mosdt breast cancers are fueled by estrogen.
The new study could change treatment, especially for younger women.
However, some women don't take tamoxifen for the full 5 years as recommended, because of side effects - the most common one is hot flashes.
It is also in competition with newer drugs called aromatase inhibitors - sold as Arimidex, Femara, Aromasin and in generic form.
They do the job with less risk of causing uterine cancer and other problems. However, they don't work well before menopause.
Researchers say Tamoxifen does have side effects; the most common is hot flashes.