7:51 p.m. | Updated The first new drug to treat lupus in more than half a century won approval from the Food and Drug Administration on Wednesday, offering a new option for people with the often debilitating immune system disease.

The drug, Benlysta, was developed by Human Genome Sciences and will be marketed by that company and GlaxoSmithKline.

The approval follows setbacks suffered by many other companies developing drugs for lupus. Doctors and patient groups said that having a new drug come to market could encourage other pharmaceutical companies to pursue the disease.

“It’s important to the field to have an approved product,’’ said Dr. David S. Pisetsky, a professor of medicine at Duke University and a scientific adviser to the Lupus Research Institute, which finances efforts to develop treatments.

The only drugs now approved to treat lupus are aspirin, approved in 1948, Plaquenil, a malaria drug, and corticosteroids. Plaquenil and corticosteroids were both approved in 1955. There are several other drugs that doctors use off-label that appear to have some effectiveness.

Approval was expected because an advisory committee to the F.D.A. had endorsed the drug in November by a 13-to-2 vote, despite reservations of many on the committee that the drug was only marginally effective.

About 11 patients must be treated for one to benefit, according to data from the clinical trials of the drug. The drug was not tested in people with the some of the most severe forms of the disease, involving active damage to the kidneys or central nervous system.

African-Americans, who have a higher incidence of the disease, did not seem to respond to treatment with Benlysta, but too few of them participated in the trial to draw definitive conclusions, according to the F.D.A. Human Genome Sciences has agreed to do another study to assess the effectiveness and safety of the drug in people of African descent.

The companies have not yet announced the price of Benlysta, but analysts expect it to be about $35,000 a year. Analysts generally expect worldwide sales to eventually exceed $2 billion a year. But sales are expected to be slow at first because insurers might balk at paying for the drug, at least until patients try less expensive alternatives.

An earlier version of this post said the price of Benlysta would be about $30,000, but the company has since issued an estimate of $35,000.