WASHINGTON (CNN) -- The Food and Drug Administration on Friday approved the first oral treatment to slow the progression of rheumatoid arthritis.

The FDA said Arava, marketed by Hoechst Marion Roussel, is not a cure, but has been shown to relieve the painful swelling and slow the joint damage associated with the disease.

Rheumatoid arthritis is an auto-immune disease that affects more than 2 million Americans. It often develops between the ages of 25 and 50.

Arava blocks production of immune cells that cause joint inflammation. In one study, the drug improved the condition of 41 percent of 480 arthritis patients.

The current treatment for rheumatoid arthritis is low doses of the cancer drug methotrexate. It has serious side effects and its potency in the body can diminish over time.

Animal studies raised concerns that Arava can cause birth defects, so a warning has been included in the labeling saying the drug should never be used by pregnant women or women of childbearing age who are not using reliable contraception.

The drug remains in the body for a long time so women who take the drug and want to get pregnant later need to take appropriate drug elimination precautions.

The drug's side effects include risk of liver toxicity, diarrhea, hair loss and rash.

Arava is expected to be available by mid-October. A daily dose will cost about $8. The first in the new class of biological drugs, Enbrel, will be reviewed by an FDA advisory committee on September 16.

The Associated Press contributed to this report.