Pfizer Inc.and Sanofi-Aventis on Wednesday separately announced their decision to cancel further examination into their experimental obesity drugs that block the cannabinoid type 1 pleasure centers in the brain, part of a class of drugs called CB-1 antagonists, because of side effects uncovered in clinical trials.

Earlier this month, Merck & Co. said it was halting all research on its obesity drug called taranabant. Pfizer, the world’s largest drugmaker, is among the last drugmakers to abandon obesity medicines that block the cannabinoid type 1 pleasure centers in the brain.

"While confident in the safety of the compound, we believe that this is the appropriate decision based on all available information regarding this class of agents, as well as recent discussions with regulatory authorities," Pfizer research chief Martin Mackay said in a statement.

The class of drugs that work by blocking the same brain receptor that makes marijuana smokers hungry have been linked to psychiatric side effects, including mood disorders and neurodegenerative disorders such as multiple sclerosis.

Sanofi-Aventis suspended European sales of its CB-1 antagonist, Acomplia after European regulators said the drug should be withdrawn from the market. Between June and August 2008 there were five suicides among participants in the trial who took the drug, compared to one among those taking a placebo. The agency had concluded that "the benefits of Acomplia no longer outweigh its risk" and there was an approximate doubling of the risk of psychiatric disorders in obese or overweight patients taking Acomplia versus a placebo. In a statement, Sanofi-Aventis said it would comply with the EMEA’s decision, but reiterated its belief that the drug could bring significant benefits to obese and overweight people.