US regulators approved the second new anti-obesity drug in 13 years, Qsymia, for use with exercise and a good diet in people who are obese or overweight with certain medical problems.

Some analysts have touted Qsymia as the next "blockbuster", akin to the best-selling cholesterol drug Lipitor, with the US market desperate for new treatments for the two-thirds of the population that is overweight or obese.

But critics fear it carries side effects that could be dangerous, like speeding up heart rates and causing birth defects, and warranted further study before being approved for widespread use.

"Qsymia, used responsibly in combination with a healthy lifestyle that includes a reduced-calorie diet and exercise, provides another treatment option for chronic weight management in Americans," said Janet Woodcock, director of the Food and Drug Administration's Center for Drug Evaluation and Research.

The drug, which was formerly known as Qnexa, is approved for use in people who are obese, meaning they have a body mass index of 30 or higher, or in people who are overweight and have at least one related condition like diabetes, high cholesterol or high blood pressure.

In two randomized, controlled trials of around 3,700 obese and overweight patients followed for one year, the highest doses of the drug were associated with an average weight loss of between 6.7 percent and 8.9 percent over a sugar pill.

"All patients received lifestyle modification that consisted of a reduced calorie diet and regular physical activity," the FDA said.

Made by California-based Vivus Pharmaceuticals, Qsymia contains phentermine and topiramate, two drugs that are already on the market for aiding weight loss and preventing seizures.

Some doctors already prescribe the combination as an off-label use for helping patients manage their weight.

The FDA warned that Qsymia "must not be used during pregnancy because it can cause harm to a fetus," as studies have shown an increased risk of babies being born with cleft palate when women taking the drug became pregnant.

It must also not be taken by patients with glaucoma or hyperthyroidism, and may increase heart rate so should be avoided by people with heart disease.

The Food and Drug Administration last month approved Belviq by the US company Arena Pharmaceuticals, making it the first new weight loss aid to hit the market since 1999.

An advisory panel to the FDA urged approval of Qsymia in February, after warning against its approval in 2010 due to safety concerns.

The FDA said the drug should be taken for an initial period of 12 weeks, and if patients have not lost at least three percent of their body weight by then, the dose may be increased.

If the patient does not achieve a loss of five percent of body weight by week 12 on the higher dose, then treatment should be discontinued, the FDA said.

Critics have raised concern about the drug's tendency to increase heart rate and the risk of irregular heartbeat.

The United States has approved and later banned a series of diet drugs, mainly over excessive risk of heart attack and stroke.

Among them are ephedra, which was banned in 2003; dexfenfluramine, a component in fen-phen that was approved in 1996 and banned in 1997; and Meridia (sibutramine), which was approved in 1997 and taken off the market in 2010.

Prior to Belviq's approval in June, the last anti-obesity drug approved in the United States was Xenical (Orlistat) by Roche in 1999.

Phentermine, approved in 1959, is also on the market for controlling appetite, and makes up one of the two key ingredients in Qsymia.

According to Jacob Warman, chief of endocrinology at The Brooklyn Hospital Center, Qsymia would be "the next blockbuster, no question."

"I think the FDA knows that the general population is hungry for a diet drug," he said in an interview, noting that Qsymia exceeds the bar for considering weight loss "significant", or higher than five percent of body weight loss.

The effect of recently approved Belviq was generally lower. People lost an average of three to 3.7 percent of their body weight after a year when compared to a placebo.