In studies conducted for F.D.A. approval, the most commonly reported side effects were bruising, pain and swelling. Dr. Derek H. Jones, a dermatologist and a principal investigator in a trial of the drug’s effectiveness, found that people with the most fat had the most swelling. It usually cleared up in 48 to 72 hours.

“That makes sense, because you’re destroying more fat cells,” said Dr. Jones, a paid consultant who will teach injectors and who treated Ms. Gallagher. In his experience, the first treatment is the most painful.

The injections will be commercially available in late summer, after doctors are trained to give them. The price for Kybella is not yet set.

Not every patient is a good candidate for this treatment, several experts said. If a patient has excess fat and loose skin, Dr. Rohrich said, “you’ll have a crepey neck and need a neck lift” after fat is reduced with the shot. Neck lifts can cost $5,000 or more.

Image Ms. Gallagher before the injections to slim down her double chin.

The drug does not target only fat cells. It will also destroy skin cells if inadvertently injected into the skin, the F.D.A. has warned.

“If you inject into the dermis, you can hurt the skin,” Dr. Rohrich said. “This drug needs to go into fat tissue to work.” Misdirected injections could even harden skin, he said.