Halaven 1 mg Injection is used in the treatment of metastatic breast cancer. It is used to treat adults with breast cancer that has spread to other parts of the body, and who have already received other anticancer medicines after the cancer has spread. Halaven 1 mg Injection is an anticancer medication. It works by interfering with the microtubule structures which help the cancer cells to divide and multiply. This slows the growth of cancer cells and eventually kills them.