An Oklahoma judge Monday blocked a new state law that would make girls under 17 get a prescription before buying emergency contraception, a limit already dropped by federal regulators.

Women's health advocates, including an Oklahoma nurse with a 15-year-old daughter, sued to keep the law from taking effect on Thursday, arguing it violated the state constitution by singling out one drug for restrictions.

In April, a federal judge struck down the the U.S. Food and Drug Administration's rule that required minors to get a prescription for the single-pill emergency contraceptive known as Plan B One-Step. The Obama administration dropped its appeal of that ruling two months later.

The Oklahoma law, which Gov. Mary Fallin signed in May, is an attempt to keep that rule in place "to protect young females," lawyers for the state argued. But a researcher who has studied emergency contraceptives told the court that the law "is unsupportable by evidence and provides no demonstrated benefit."

When used as directed, the drug is "as effective and safe for minors as it is for adults," Dr. Elizabeth Raymond wrote in support of the plaintiffs.