Merck & Co's experimental obesity drug taranabant did not show impressive effectiveness in a late-stage trial and its side effects were worse than expected. The pill is one of Merck's most important experimental products, and some analysts have predicted it could garner annual sales of $1 billion or more, if approved by the U.S. Food and Drug Administration. It belongs to the same family of medicines as rimonabant, a Sanofi-Aventis drug that was rejected by U.S. regulators after it was linked to suicidal thoughts and depression. Both drugs work by blocking cannabinoid receptors in the brain. They are the same receptors that make people hungry when smoking marijuana. This news comes from an abstract, or abbreviated summary, of Merck's two-year study. Full data from the two-year trial will be formally presented late this month at the annual meeting of the American College of Cardiology in Chicago. The 2 milligram dose of the Merck drug -- the maximum dose expected to be used in any approved product -- failed to produce the desired 5 percent weight loss, compared to placebo. While the highest dose of the Merck drug used in the trial, 4 milligrams, did achieve the desired 5 percent weight loss, it was associated with a considerably higher incidence of psychiatric side effects, said analysts. Stay tuned. Merck has great scientists and is very aggressive in getting its drugs to market. If this drug will work, it will be in your medicine chests before you know it.