Over the past 20 years, hundreds of thousands of women with breast cancer have taken the drug Herceptin, typically for a year or more. The medicine, used to treat an aggressive form of the disease, is credited with saving many lives, but it also has some tough side effects, particularly damage to the heart.

A large new study that followed thousands of women with early-stage breast cancer for a median of more than five years has found that those treated with Herceptin for only six months did just as well as those who got it for a year — and they suffered fewer side effects.

The shorter regimen also saved money. A yearlong course of the drug costs $76,700.

Dr. Bruce E. Johnson, president of the American Society of Clinical Oncology, said the study’s findings should change treatment for women who have early-stage breast cancer treatable by Herceptin. He was not involved in the research, which was done in Britain and will be presented at a meeting of the oncology society next month.

“A lot of people will likely adopt a shorter regimen based on this finding,” Dr. Johnson said.

Cancer specialists say the study offers not only important news for women’s health, but also highlights a type of research that pharmaceutical companies almost never do and that is also critically important for cancer drugs that can often be quite toxic.