FDA approves Pfizer drug to treat rheumatoid arthritis Tue Nov 6, 2012 4:22pm EST (Reuters) - The U.S. Food and Drug Administration said on Tuesday it has approved a new drug to treat rheumatoid arthritis developed by Pfizer Inc, two weeks ahead of the date the agency was expected to complete its review. The drug, Xeljanz, known generically as tofacitinib, is approved to treat adults with moderate to severe arthritis who cannot take, or have an inadequate response to, methotrexate, a standard rheumatoid arthritis treatment. Rheumatoid arthritis is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue, causing inflammation and pain in the joints. It affects an estimated 1.5 million Americans, the FDA said. Pfizer's drug works by blocking molecules called Janus kinases which are implicated in joint inflammation. The drug was associated with an increased risk of serious infections, tuberculosis, cancers and lymphoma. It carries a boxed warning, the most serious available, related to its side effects. Reporting by Toni Clarke; Editing by Gary Hill