Signs and symptoms of multiple myeloma can vary and, early in the disease, there may be none.

When signs and symptoms do occur, they can include:

• Bone pain, especially in your spine or chest
• Nausea
• Constipation
• Loss of appetite
• Mental fogginess or confusion
• Fatigue
• Frequent infections
• Weight loss
• Weakness or numbness in your legs
• Excessive thirst

It's not clear what causes myeloma.

Doctors know that myeloma begins with one abnormal plasma cell in your bone marrow — the soft, blood-producing tissue that fills in the center of most of your bones. The abnormal cell multiplies rapidly.

Because cancer cells don't mature and then die as normal cells do, they accumulate, eventually overwhelming the production of healthy cells. In the bone marrow, myeloma cells crowd out healthy white blood cells and red blood cells, leading to fatigue and an inability to fight infections.

The myeloma cells continue trying to produce antibodies, as healthy plasma cells do, but the myeloma cells produce abnormal antibodies that the body can't use. Instead, the abnormal antibodies (monoclonal proteins, or M proteins) build up in the body and cause problems such as damage to the kidneys. Cancer cells can also cause damage to the bones that increases the risk of broken bones.

A connection with MGUS

Multiple myeloma almost always starts out as a relatively benign condition called monoclonal gammopathy of undetermined significance (MGUS).

In the United States, about 3 percent of people older than age 50 have MGUS. Each year, about 1 percent of people with MGUS develop multiple myeloma or a related cancer.

MGUS, like multiple myeloma, is marked by the presence of M proteins — produced by abnormal plasma cells — in your blood. However, in MGUS, the levels of M proteins are lower and no damage to the body occurs.

Factors that may increase your risk of multiple myeloma include:

• Increasing age. Your risk of multiple myeloma increases as you age, with most people diagnosed in their mid-60s.
• Male sex. Men are more likely to develop the disease than are women.
• Black race. Black people are about twice as likely to develop multiple myeloma as are white people.
• Family history of multiple myeloma. If a brother, sister or parent has multiple myeloma, you have an increased risk of the disease.
• Personal history of a monoclonal gammopathy of undetermined significance (MGUS). Every year 1 percent of the people with MGUS in the United States develop multiple myeloma.

Complications of multiple myeloma include:

• Frequent infections. Myeloma cells inhibit your body's ability to fight infections.
• Bone problems. Multiple myeloma can also affect your bones, leading to bone pain, thinning bones and broken bones.
• Reduced kidney function. Multiple myeloma may cause problems with kidney function, including kidney failure. Higher calcium levels in the blood related to eroding bones can interfere with your kidneys' ability to filter your blood's waste. The proteins produced by the myeloma cells can cause similar problems.
• Low red blood cell count (anemia). As myeloma cells crowd out normal blood cells, multiple myeloma can also cause anemia and other blood problems.