The drug combines two older ones — one that suppresses appetite and one that leads to more satisfaction after eating.
"We now know there are multiple pathways that determine how much energy we take in every day," Dr. Tim Garvey, of the University of Alabama at Birmingham, told The Associated Press.
"If you intervene on one pathway it's hard to make much of a difference, you really need to attack multiple mechanisms to get a pronounced effect."
The Food and Drug Administration approved Arena Pharmaceutical's "Belviq" weight-loss pill less than one month ago.
In the past, unapproved diet pills and doctor-prescribed drug cocktails aimed at weight loss have been linked to heart problems.