In most states, anyone age 17 and over can get the pill without a prescription, as a result of a 2009 decision by the U.S. Food and Drug Administration.

But the new research suggests some pharmacies may not always stock the medication, also known as Plan B, or workers may think only older patients can get it over the counter - and teens may have trouble accessing emergency contraception as a result.

"This is a delicate topic, and it could be something that's very sensitive for the teenager calling," said Dr. Tracey Wilkinson, a pediatrician at Boston Medical Center and the lead author of the study published in the journal Pediatrics.

"If you don't give the right information or you're not willing to have a discussion about how to help get the medication, the adolescent might just give up," she told Reuters Health.

Research assistants working with Wilkinson called every pharmacy in five different cities, pretending to be either a 17-year-old girl or the doctor of one, looking to get the morning-after pill.

Altogether, the researchers made calls to 943 chain and independent pharmacies in Nashville, Philadelphia, Cleveland, Austin and Portland, Oregon.

Most of the pharmacies - about 80 percent - told both girls and doctors that they had emergency contraception in stock. But even in those cases, one in five of the teen callers was told there was no way she could get the medication when she gave her age.

When the morning-after pill was not available, more than one-third of pharmacies did not suggest any other options for how girls could get it.

"I had heard anecdotal stories from my patients and also from other practitioners about adolescents really having difficulty obtaining emergency contraception," Wilkinson said.

But when one in five girls couldn't get the medication that was in stock, "I was shocked," she said.

"They were basically being told, ‘You can't get it at all,' which is not true but also scary, because you would think maybe a teenager wouldn't call another pharmacy."

Getting the pill quickly is important because it becomes less effective for every day that passes after unprotected sex.

Study callers who were told they could get the pill also asked pharmacy workers what the minimum age was for over-the-counter access. And in that case, only about 60 percent gave the correct answer to girls or doctors -- the rest typically thought only older women did not need a prescription.

The results suggest little has changed over the years when it comes to education about and access to emergency contraception, said one reproductive health researcher not involved in the new study.

James Trussell from Princeton University in New Jersey said he and his colleagues did a similar study over a decade ago, and found that even providers who said they stocked emergency contraception were not always able to help women get the pills.

Now, he told Reuters Health, "You're getting results that are not at all dissimilar, although this time it appears to be over not having it in stock or being completely uninformed about what the rules are regarding age."

Researchers recently suggested that girls younger than 17 could safely use the morning-after pill without a prescription -- an idea that has been shot down by the Department of Health and Human Services. SOURCE: bit.ly/jsoh2P Pediatrics, online March 26, 2012.

(Editing by Christine Soares and Michele Gershberg; Desking by Lisa Shumaker)

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