The Food and Drug Administration on Friday approved a new type of drug that combines the widely used breast cancer medicine Herceptin with a powerful toxin to more effectively kill cancer cells while potentially reducing side effects.

The drug, which will be called Kadcyla but was known as T-DM1 during its development, extended the median survival of women with advanced breast cancer by nearly half a year in a clinical trial.

Genentech, which developed the drug, said it would cost about $9,800 a month, or $94,000 for a typical course of treatment. That is about twice the price of Herceptin itself, which is also made by Genentech, but it is similar to the price of some other new cancer drugs.

Kadcyla, which the company said could be available within days, is one of the first successful examples of a new class of drugs that link toxins to proteins known as monoclonal antibodies. The antibodies latch onto tumors and deliver the toxic payload. Because the toxin is not activated until it reaches the tumor, healthy cells are spared and some side effects are avoided.