The makers of Ambien and similar sleeping pills will be required to lower the dosage of their drugs due to studies suggesting patients face a higher risk of injury due to morning drowsiness, the Food and Drug Administration said Thursday. The agency said Thursday that new research shows that the drugs remain in the bloodstream at levels high enough to interfere with morning driving, which increases the risk of car accidents. Regulators are ordering drug manufacturers to cut the dose of the medications in half for women, who process the drug more slowly. Doses will be lowered from 10 milligrams to 5 milligrams for regular products, and 12.5 milligrams to 6.25 milligrams for extended-release formulations.