The National Cancer Institute says to stop the studies two years early because the drug is working twice as well as expected.
For seven years, Herceptin has been used in treating late stage breast cancers. But this research involved 3,300 women with early stage breast cancer that had spread to the lymph nodes.
The studies found that when given after surgery and along with standard chemotherapy, Herceptin reduced recurrences of breast cancer by 52%. And then after four years, only 15% of patients had a relapse.
"This is certainly one of the biggest discoveries in the clinical care of breast cancer patients in more than 20 years," says Edith Perez, a Mayo Clinic lead researcher.
Even though the trials are now over, researchers say they'll continue to monitor patients for side effects since Herceptin carries a small risk of heart trouble.