WASHINGTON -- The government has rejected over-the-counter sales of morning-after birth control for now.

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The Food and Drug Administration cited concern about young teenagers' use of the pills.

The FDA has been under intense political pressure from both sides on whether to lift the prescription requirement for Plan B, Barr Laboratories' version of emergency contraception.

Backers argue that easier access to the pills could prevent thousands of abortions. Conservatives contend that could encourage unsafe sex.

The FDA said there isn't yet proof that teens younger than 16 could safely use the pills without a doctor's guidance.

The government has left the door open for Barr Laboratories to try again. It must provide data showing young teens could use the pills safely without a prescription, or offer details on how the product could be sold over-the-counter and with prescriptions for those 16 and under.

Plan B has been shown to reduce the risk of pregnancy by 89 percent after having unprotected sex. Its effectiveness wanes the longer it is between having sex and taking the medication. Plan B is most effective when taken in the first 24 hours after intercourse, according to Barr.