Overweight Americans turning to prescription medication for help lost another option Thursday, as the FDA rejected the diet drug Qnexa. That makes Qnexa the third weight-loss drug this month to be spurned by the group, which has recently heightened its scrutiny of diet pills due to safety concerns. The FDA declined to approve lorcaserin and Meridia this month as well. This setback is especially difficult, though, because Qnexa was shown to be much more effective than others of its type in trials; test patients shed about 10.6 percent of their weight with the drug. The FDA doesn’t state its reasons for rejection, but critics say that its decision could discourage drug makers to enter the field again.