ABC NEWS

ABC News' Sharon Alfonsi and Cathy Becker report:

The diet drug Qsymia that was approved by the U.S. Food and Drug Administration earlier this year is available by prescription starting today.

Data presented by the manufacturer of the drug, Vivus, showed that it helped patients lose about 10 percent of their body weight. Qsymia is a combination of two medications that have long been used in weight management. One is an appetite suppressant and the other reduces cravings for food.

An advisory panel voted 20-2 to approve the drug in February, the first time the FDA voted to approve a weight-loss drug in more than 13 years. It's the FDA's latest move to give doctors and their patients more tools to fight excessive weight gain as obesity rates continue to bulge in the U.S. and around the world.

Contrary to other weight-loss products, the FDA has indicated Qsymia as an approved product for female patients of child-bearing age. Patients in that category can use the drug as long as they have a negative pregnancy test before and while taking it, and use effective birth control consistently while taking Qsymia.

"This is a medication that studies show is one of the most effective medicines we've ever seen for controlling weight," said Dr. Louis Aronne of New York Presbyterian Hospital and Weill Cornell Medical Center.

About one-third of Americans are obese, and many have chronic, expensive medical conditions as a result, such as heart disease, diabetes and arthritis. Until recently, the array of available options has been frustratingly sparse for many doctors and their patients: diet, exercise and, for those overweight enough to qualify, bariatric surgery.

Weight loss was a struggle for Gwendolyn Barton, a 57-year-old grandmother, who tried just about everything to lose those extra pounds.

"I've done diet, I've done exercise and I've done Weight Watchers. And none of it has ever helped me," she said.

Barton began using Qsymia as part of a clinical trial and in one year she dropped 50 pounds.

"My blood pressure started to go down. I felt much better," said Barton. "I was able to do things with my grandkids…" she added.

Still, doctors are mixed in their concern over the potential for side effects, particularly in light of the history of diet drugs, such as fen-phen, approved by the FDA, then withdrawn from the market over concerns about heart risks and other dangerous side effects.

But losing weight is not all about popping a pill. The drug has to be used in conjunction with a healthy diet and exercise.

For Barton, Qsymia has been a life changer.

"I went from a size 14 to a size six. It made me feel great," said Barton.

ABC News' Carrie Gann contributed to this report.