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By Maggie Fox and Sossy Dombourian

Doctors hoping that it might be possible to take it a little easy on treating babies with antibiotics got bad news Wednesday: A shorter treatment course didn’t work as well as 10 full days of drugs.

The experiment involved babies and young toddlers with ear infection. The hope was that doctors could cut in half the usual 10 day course of antibiotics without doing any harm.

But the babies and toddlers who got just five days of treatment were half as likely to be cured as those who got all 10 days.

Related: A Third of Antibiotic Prescriptions Are Just Wrong

"You really need 10 full days of antibiotics to clear a typical bacterial ear infection, in an infant or toddler,” said Dr. Tanya Altmann of the University of California Los Angeles and Calabasas Pediatrics, who was not involved in the study.

Dr. Alejandro Hoberman of the University of Pittsburgh School of Medicine and Children's Hospital of Pittsburgh and colleagues tested more than 500 babies aged 6 months to 2 years who had ear infections, giving them either five or 10 days of antibiotics.

They stopped the trial early when it became clear that the shorter course wasn’t working.

"You really need 10 full days of antibiotics to clear a typical bacterial ear infection, in an infant or toddler."

"Clinical failure was observed in a greater percentage of children treated with amoxicillin–clavulanate for five days than of those treated for 10 days," they wrote in the New England Journal of Medicine.

The infection kept raging, or came right back, in a third of the kids who got the five-day course, compared to 16 percent of the kids who got the 10-day course.

Jocelyn Maynes and her son BronsonNBC News

Luckily, the babies and toddlers treated for five days were not more likely to grow drug-resistant bacteria in their bodies. And the rates of diarrhea and diaper rash were about the same in both groups.

Most kids who have ear infections do not need antibiotics. The Centers for Disease Control and Prevention, the American Academy of Pediatrics and other groups are clear on that. Most ear infections are caused by viruses and antibiotics won’t help.

Related: Doctors Advise Against Antibiotics For Ear Infections

So doctors are reluctant to prescribe them, said Altmann.

"With older kids you can ask them how they're feeling and it's sometimes okay to wait a day or two," she told NBC News.

But with babies and kids under 2, it’s a little trickier. They can’t tell parents or doctors clearly how they are feeling, and pediatricians tend to err on the side of giving antibiotics more quickly when a child so small has the telltale signs of a serious middle ear infection: a high temperature, pus in the ears, symptoms that last, loss of appetite, and cranky behavior.

"You just don't like seeing your kid sick or in pain or especially with a fever," said Jocelyn Maynes, a Calabasas realtor whose son Bronson is 8 months old. "I feel awful. I want to be able to do something for him."

Dr. Tanya Altmann of UCLA checks Bronson Maynes, 8 months old, for signs of an ear infection.NBC News

But giving antibiotics can be harmful. It can kill off the "good" bacteria in a child's body and frequent dosing of antibiotics in early childhood is linked with obesity. In addition, the more times anyone takes antibiotics, the more chances there are for drug-resistant superbugs to evolve.

Altmann has seen it happen, especially when kids don’t finish a full course of antibiotics.

Related: Antibiotic Gel May be Better for Ear Infections

"Sometimes the infection comes back, and sometimes when it comes back, it’s resistant to that first antibiotic the child was on," she said. "Then you have to turn to a stronger antibiotic for a longer period of time to clear up that original infection."

Maynes says she's okay with knowing that is Bronson has an ear infection, she'll have to give him the full 10 days of treatment.

"If five days isn't working, then 10 days is preferable," she said.