In clinical studies, according to published reports, patients on Qsymia lost more weight, dropping from an average 227 pounds to 204; on Belviq, patients went from an average 220 pounds to 207.
"But you must bear in mind they were not studied head to head, so you can't make firm conclusions that one is necessarily better than the other," said Allegheny General Hospital internist Marc Itskowitz, whose practice includes many obese or overweight patients.
Both drugs have potential side effects.
Most common side effects of Belviq are headache, fatigue, nausea, dry mouth, dizziness, and constipation. The FDA also has ordered manufacturer Arena Pharmaceuticals to conduct six studies once the drug reaches the market to determine whether it causes heart attacks or stroke.
Qsymia (pronounced kyoo-SIM-ee-uh) caused some patients in the clinical trial to suffer increased heart rate and metabolic acidosis, a condition that can lead to fatigue, hyperventilation and anorexia. It must not be used during pregnancy because it can damage a fetus; nor can it be used in patients with glaucoma or hyperthyroidism. Other common side effects include tingling of hands and feet, dizziness, altered taste, insomnia, constipation, and dry mouth.
The pharmaceutical industry's track record on diet pills has been troubled. The drug combination known as fen-phen was withdrawn in 1997 after more than 30 percent of patients developed heart valve problems that could lead to heart failure or death. Meridia was pulled in 2010 after being linked to heart problems.
The FDA approved both drugs for use in adults with a body mass index of 30 or more, the definition of obese, or adults with a BMI of 27 or greater (overweight) who have at least one weight-related condition such as high blood pressure or type 2 diabetes. That does not mean, however, that doctors can't prescribe them for patients with lesser BMIs.