Dactinomycin is a chemotherapy agent. It is used to treat several different types of cancer. It is administered intravenously.

The most common side effects are suppression of the bone marrow, vomiting, mouth ulcers, hair loss, liver damage, infections and muscle pain.

Although it's mechanism of action is not well understood, it is believed it interferes with the function of RNA.

Dactinomycin at Wikipedia