WASHINGTON — Kythera Biopharmaceuticals Inc. won approval for a first-of-its-kind double-chin treatment.

The drug, called Kybella, is injected into the fat below the chin and destroys the fat cells, the Food and Drug Administration said in a statement. The agency warned that Kybella can also kill skin cells if inadvertently injected into the skin.

Kybella is a version of deoxycholic acid, a molecule that occurs naturally in the body to help destroy fat. Patients may receive as many as 50 injections in a single treatment, the FDA said. Injectable drugs such as Allergan Inc.’s Botox and dermal fillers aren’t approved to fix fat and loose skin under the chin.

“It is important to remember that Kybella is only approved for the treatment of fat occurring below the chin, and it is not known if Kybella is safe or effective for treatment outside of this area,” Amy Egan, deputy director of the Office of Drug Evaluation III at the FDA, said in the statement.

Serious side effects include nerve injury in the jaw that can cause an uneven smile or facial muscle weakness and trouble swallowing. The most common side effects of Kybella include swelling, bruising, pain, numbness, redness and areas of hardness in the treatment area.