The field of big drug makers racing to produce an effective weight-loss drug not only is narrowing, it’s almost drying up.

In the last month alone, the U.S. Food and Drug Administration has rejected approval for two new drugs and forced the withdrawal of a third that had been on the market for more than a decade. And in the last few years, several of the pharmaceutical industry’s biggest names — including Abbott Laboratories, Merck & Co. and Sanofi-Aventis — have tried and failed to take anti-obesity and weight-loss drugs to market. The F.D.A. hasn’t approved a prescription diet pill since 1999, when it gave a nod to Roche’s Xenical, now the only weight-management drug approved for long-term use.

The F.D.A. hasn’t acted without its reasons — but experts say pharmaceutical companies are increasingly discouraged from working on drugs for obesity, one of the nation’s biggest health problems. One-third of Americans are obese, and another third are overweight — conditions that lead to cancer, diabetes and heart disease.

One new diet drug — Contrave, developed by La Jolla, Calif.,-based Orexigen Therapeutics — is scheduled for an F.D.A. commission’s consideration in December. If it fails to win approval, expect to see the pharmaceutical industry “put the brakes on all obesity drug development for a decade,” Dr. W. Timothy Garvey, chairman of the department of nutrition sciences at the University of Alabama at Birmingham, told the New York Times.

Garvey knows the pharmaceutical industry’s disappointment firsthand. He was an investigator in the clinical trials of Qnexa, produced by Mountain View, Calif.,-based Vivus, Inc., and widely touted as the most promising weight-loss drug in more than a decade. The F.D.A. declined to approve Qnexa last month because of concerns about birth defects and heart problems. Two weeks later, the agency rejected another new drug, Lorcaserin, produced by Arena Pharmaceuticals in San Diego, because it caused tumors in rats. Both companies say they’re hopeful that they’ll produce information that addresses the F.D.A.’s concerns and wins their drugs approval next year.

Even drugs out on the market for some time have experienced significant setbacks. F.D.A. oversight resulted in the Oct. 8 recall of Meridia, produced by Abbott and sold for 13 years. Federal officials cited the risk of heart attacks and strokes for certain patients.

Aside from Contrave, Media Salad reporters are watching a drug developed by Novo Nordisk A/S to treat Type 2 diabetes. The Danish company headquartered in Copenhagen is the world’s largest maker of insulin, controlling 52 percent of the world’s market. In January, Novo Nordisk won F.D.A. approval for Victoza in the treatment of diabetes. Consumers who used the drug have reported significant weight loss — prompting Novo Nordisk to conduct late-stage studies of Victoza as an antiobesity agent.

Victoza is a synthetic version of the naturally occurring hormone GLP-1. The hormone is produced after eating and prompts the pancreas to make insulin.

Novo said in October that it expects annual Victoza sales to reach $1 billion by 2012 — roughly two years sooner than expected. And when speaking with reporters in the last week, Chief Executive Officer Lars Sorensen said annual sales of $2.5 billion later this decade are “within reach” — perhaps indicating his confidence that Novo will win F.D.A. approval to extend the Victoza label as a treatment for obesity.

Given that the F.D.A. already has approved Victoza for market, it’ll be interesting to see whether it approves the label’s extension – especially because the agency has backed itself into a corner, according to Morgan Downey, an advocate for obese people who also consults with pharmaceutical companies. Downey, a harsh critic of the F.D.A.’s recent drug rejections, said the agency has behaved as if it believes an effective drug like Victoza would be overused, resulting in unanticipated, adverse side effects, and that less effective drugs aren’t worth any risks.

“The F.D.A. has gotten itself into a real knot,” Mr. Downey told the New York Times. “They can’t approve a very effective drug, and they can’t approve a modest drug.”

Meanwhile, Novo’s weight-loss trials for Victoza continue.