About 3 to 10 percent of infants develop an infantile hemangioma, the researchers say. These red, rubbery-looking growths are caused by the abnormal buildup of blood vessels.

Although harmless, they can cause physical and psychological harm, depending on their location.

"Propranolol is the first and only medical treatment approved to treat infantile hemangioma," said lead study author Dr. Christine Leaute-Labreze, a pediatric dermatologist at the University of Bordeaux in France.

She said the results of her study were key to U.S. Food and Drug Administration approval of propranolol for the treatment of infantile hemangioma in 2014. The drug -- a so-called beta-blocker used to treat high blood pressure and angina -- works by starving the hemangioma of blood.

This stops its growth, and eventually the cells in the birthmark die, Leaute-Labreze explained.

In the current study, propranolol was taken by mouth for six months.

Dr. Ana Duarte, director of pediatric dermatology at Miami Children's Hospital in Florida, said early treatment of infantile hemangioma is needed to prevent permanent disfigurement and scarring.

"If the tumor occurs near the eyes, nose, mouth or ear, there can be long-term functional problems, so it is important to intervene early," Duarte said.

She welcomed FDA approval of propranolol for the condition.

"In 2008, I was treating them with corticosteroids, which had many side effects," Duarte said. "I really welcomed this new treatment. It completely changed the way I treat my patients."

Without treatment, these birthmarks usually fade or flatten out within nine or 10 years, Leaute-Labreze said.

For the study, published Feb. 19 in the New England Journal of Medicine, the researchers identified 460 infants 1 to 5 months old with infantile hemangioma. They were randomly assigned to take one of four doses of propranolol, or a placebo.

After 24 weeks of treatment, the most effective dose of propranolol was 3 milligrams per kilogram of body weight daily, the researchers found.

Among infants receiving that dose, 60 percent had their birthmark disappear or nearly disappear, compared with 4 percent of babies receiving a placebo, Leaute-Labreze's team found.

In all, 88 percent of infants treated with propranolol showed improvement by the fifth week, compared with 5 percent of the infants given the placebo.

The researchers added that one in 10 babies getting propranolol needed another round of therapy.

Side effects of propranolol -- high blood sugar, high blood pressure, slow heartbeat or breathing difficulty -- were rare, the researchers said. And the rate of side effects did not differ significantly between the propranolol group and the babies receiving a placebo, they noted.

The study was funded by Pierre Fabre Dermatologie, the French drug company that makes propranolol for infants.