* Approved for use after other treatments fail
* Marks seventh disease for which Humira has U.S. approval
* Condition affects about 620,000 Americans - NIH
* Abbott shares down nearly 1 percent
Sept 28 (Reuters) - U.S health regulators on Friday approved Abbott Laboratories Inc's blockbuster rheumatoid arthritis drug Humira for the treatment of moderate to severe ulcerative colitis, a form of inflammatory bowel disease.
The U.S. Food and Drug Administration approved Humira for use by adults to help control the chronic condition once immunosuppressant medicines, such as corticosteroids, have proved ineffective.
Humira, by far Abbott's biggest product with annual sales of about $8 billion and still growing, is already approved to treat a number of inflammatory diseases. In addition to rheumatoid arthritis, the medicine is approved for psoriatic arthritis, ankylosing spondylitis, Crohn's disease, plaque psoriasis and juvenile idiopathic arthritis.
Morningstar analyst Damien Conover said the latest approval "will help to continue building the franchise" and could eventually add annual revenue approaching $500 million.
Ulcerative colitis, a chronic disease that causes inflammation and ulcers in the inner lining of the large intestine, affects about 620,000 Americans, according to the National Institutes of Health.
"Each patient with ulcerative colitis experiences the disease differently, and treatment must be adjusted to meet each individual's needs," Donna Griebel, of the FDA's Division of Gastroenterology and Inborn Errors Products, said in a statement.
"Today's approval provides an important new treatment option for patients who have had an inadequate response to conventional therapy," she added.
Humira should only continue to be used in patients who have shown evidence of clinical remission after eight weeks of therapy, the FDA said.
Abbott shares were down 65 cents, or nearly 1 percent, at $68.67 on the New York Stock Exchange.