The pill, called Qsymia, was approved for patients who are overweight or obese and also have at least one weight-related condition such as high blood pressure, diabetes or high cholesterol.
The drug offers hope for those who have failed to lose weight in other ways, but its path to approval also underscores how difficult it has been for drugmakers to find obesity treatments that are safe and effective.
The drug's maker, Vivus Inc., said it plans to bring the drug to market in the fourth quarter of this year. It hasn't yet decided what the pills will cost.
In testing, the drug made led patients to lose more weight than two other weight-loss pills recently review by the FDA. Patients taking Qsymia for a year lost 6.7 percent of their body weight in one study and 8.9 percent in another study, the FDA said. The company said patients on the strongest formulation tested lost nearly 11 percent of their weight.
Despite its impressive performance in clinical trials, Qsymia is not exactly a scientific breakthrough. The drug is actually a combination of two older drugs long known to help with weight loss: phentermine and topirimate.
Phentermine is a stimulant that suppresses the appetite, and has long been used for short-term weight loss. Topiramate is an anticonvulsant, sold by Johnson & Johnson as Topamax, that makes people feel more satisfied after eating.
Researchers say the innovation of Qsymia lies in targeting more than one of the brain signals that drive people to overeat.
"We now know there are multiple pathways that determine how much energy we take in every day," said Dr. Tim Garvey of the University of Alabama at Birmingham. "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."
Garvey helped conduct several pivotal trials of the drug.
Qsymia is the second weight-loss drug approved by the FDA in less than a month, following Arena Pharmaceutical's pill Belviq in late June. Previously the agency had not approved a new drug for long-term weight loss since 1999.
With U.S. obesity rates nearing 35 percent of the adult population, many doctors have called on the FDA to approve new weight-loss treatments. But a long line of prescription diet pills have been associated with dangerous side effects, particularly heart problems.
In 1997, the popular diet drug combination fen-phen was linked to heart valve damage. The cocktail of phentermine and fenfluramine was a popular weight-loss combination prescribed by doctors, though it was never approved by the FDA. Fenfluramine was eventually withdrawn from the market.
Other safety failures for diet pills have continued to pile up in recent years. In 2010, Abbott Laboratories withdrew its drug Meridia after a study showed it increased risks for heart attack and stroke.
The FDA's approvals of Qsymia and Belviq suggest a new willingness to make weight-loss medications available, even in the face of lingering safety issues.
The FDA initially rejected Vivus' drug in 2010 over concerns that it can cause birth defects if taken by pregnant women. The agency laid out a risk-management plan Tuesday specifically designed to minimize the chance of the women becoming pregnant while using the drug. It recommends that women of childbearing age test negative for pregnancy before starting the drug and take a monthly pregnancy test while taking it.
The agency also said patients with recent or unstable heart disease or stroke aren't good candidates for the drug because its effect on heart rates in those patients is not known. Vivus has to do studies of the heart effects of Qsymia, the FDA said.
Analysts estimate the new pill could garner more than $1 billion in sales by 2016, though Mountain View, Calif.-based Vivus plans a slow rollout.
The pill will launch with a relatively small sales force of 150 representatives. Company executives say their initial marketing efforts will focus on obesity specialists, not general doctors.
"We're going to have to grow our sales organization in order to support the primary care market," Vivus President Peter Tam said in an interview with the Associated Press.
Vivus had originally planned to market the drug under the brand name Qnexa. However, FDA regulators ordered the company to change the name to avoid potential confusion with similar-sounding drugs.
Rival Arena Pharmaceuticals Inc. of San Diego plans to start selling Belviq early next year. A third California drugmaker, Orexigen Therapeutics Inc., is still running clinical trials of its product, Contrave, and is working toward an FDA approval date in 2014.