Bortezomib is used to treat people with multiple myeloma (a type of cancer of the bone marrow) who have already been treated with at least one other medication. Bortezomib is also used to treat people with mantle cell lymphoma (a fast-growing cancer that begins in the cells of the immune system) who have already been treated with at least one other medication. Bortezomib is in a class of medications called antineoplastic agents. It works by killing cancer cells.Bortezomib comes as a solution (liquid) to inject into a vein. Bortezomib is given by a doctor or nurse in a medical office or clinic. It is usually given on a rotating schedule that alternates 2 weeks when bortezomib is given twice a week with 10 days when the medication is not given. During the weeks that bortezomib is given, doses will always be at least 72 hours apart. The rotating schedule may be followed for up to eight cycles. After that, your doctor may decide to continue your treatment, but you will receive bortezomib less often.