From pharmaceutical giants to tiny start-ups, the drug industry is spending billions of dollars developing obesity drugs. Drug makers say they are tackling fat in response to public health warnings of a national obesity epidemic -- one that accounts for more than $100 billion in annual medical bills.

As it becomes more and more difficult to identify blockbusters, many companies see few other prospective best sellers in their research pipelines.

Dr. Bergman said that concerns about oversubscription and abuse were not reason enough to discourage the development and approval of obesity drugs. The growing ranks of the obese are simply too alarming a problem, he said. "We have to do something to save them."

Scientists believe that the body's heavily defended feeding system is the reason diets tend to fail and may also make it difficult for the industry to find a safe and effective diet drug.