In a stem cell transplant, the patient gets high-dose chemotherapy to kill the cells in the bone marrow. Then the patient receives new, healthy blood-forming stem cells. When stem cell transplants were first developed, the new stem cells came from bone marrow, and so this was known as a bone marrow transplant. Now, stem cells are more often collected from blood (a peripheral blood stem cell transplant).

Stem cell transplant is commonly used to treat multiple myeloma. Before the transplant, drug treatment is used to reduce the number of myeloma cells in the patients body. (See Drug Therapy for Multiple Myeloma).)

Stem cell transplants (SCT) can be autologous or allogeneic.

For an autologous stem cell transplant, the patients own stem cells are removed from the bone marrow or peripheral blood before the transplant. The cells are stored until they are needed for the transplant. Then, the person with myeloma gets treatment such as high-dose chemotherapy, sometimes with radiation, to kill the cancer cells. When this is complete, their stored stem cells are given back into their blood through a vein.

This type of transplant is a standard treatment for patients with multiple myeloma. Although an autologous transplant can make the myeloma go away for a time (even years), it doesnt cure the cancer, and often the myeloma returns.

Some doctors recommend that patients with multiple myeloma have 2 autologous transplants, 6 to 12 months apart. This approach is called tandem transplant. Studies show that this may help some patients more than a single transplant. The drawback is that it causes more side effects and as a result can be riskier.

In an allogeneic stem cell transplant, the patient gets blood-forming stem cells from another person the donor. The best treatment results occur when the donors cells are closely matched to the patients cell type and the donor is closely related to the patient, such as a brother or sister. Allogeneic transplants are much riskier than autologous transplants, but they may be better at fighting the cancer. Thats because transplanted (donor) cells may actually help destroy myeloma cells. This is called a graft vs. tumor effect. In studies of multiple myeloma patients, those who got allogeneic transplants often did worse in the short term than those who got autologous transplants. At this time, allogeneic transplants are not considered a standard treatment for myeloma, but may be done as a part of a clinical trial.

The early side effects from a stem cell transplant (SCT) are similar to those from chemotherapy and radiation, only more severe. One of the most serious side effects is low blood counts, which can lead to risks of serious infections and bleeding.

The most serious side effect from allogeneic transplants is graft-versus-host disease (or GVHD). This occurs when the new immune cells (from the donor) see the patients tissues as foreign and attack them. GVHD can affect any part of the body and can be life threatening.

More here:
Stem Cell Transplant for Multiple Myeloma - American Cancer Society