The 10-year trial -- reported in the October issue of Cell Metabolism -- measured the weight loss of 832 obese people, and while they tolerated the drug well, they only lost an average of 7.5 pounds while the placebo group lost an average of 4 pounds.
"People did lose a statistically significant amount of weight, but not clinically meaningful," said lead researcher Dr. Steven B. Heymsfield, executive director of clinical sciences at Merck. "The weight loss was about 3 pounds above the placebo group, but that has no commercial viability for a drug company."
MK-0557's power was supposed to lie in its ability to regulate hunger by blocking the NPY5R receptor, but while the drug caused significant weight loss in animals -- and even completely blocked the receptor in humans -- the study results seem to suggest NPY5R is not a major factor in hunger regulation.
"Maybe it's not a main pathway," Heymsfield said. He also suggested that multiple pathways might regulate food intake and, "if one system shuts down, other systems compensate for it."
Some experts say that the real flaw of the drug is it is indicative of looking for "magic bullet" cures for things like weight loss.
"In my view, this trial suggests not that a cocktail of drugs will be needed, but that for the most part, drugs are not the right answer at all," said Dr. David L. Katz, an associate professor of public health and director of the Prevention Research Center at Yale University School of Medicine. "Though rare cases of obesity may warrant medication as part of a comprehensive treatment plan, the hope that drugs will save most of us the trouble of addressing weight control through lifestyle practices is misplaced.
"The causes of epidemic obesity are all around us and largely modifiable whenever we muster the will," Katz said. "The relative failure of MK-0557 will perhaps encourage us to focus less on attempts to block receptors in our brains, and more on using our brains to unblock opportunities for more healthful eating and activity in the schools, work sites, and communities we frequent each day."
Natural health advocate and "Food Timing Diet" author Mike Adams agreed.
"No pill is going to magically melt pounds away, but using nutritious foods to balance your hunger, exercising, and having realistic expectations about weight loss can help you shed excess weight," he said. "When you eat can be as important as what you eat. You can control your appetite by eating the right foods at the right time, and still lose weight."