By Elise Viebeck - 12/20/12 07:34 PM EST
"Plan B" works to prevent pregnancy in the days after unprotected sex by stopping ovulation and inhibiting the movement of sperm.
For years, the Food and Drug Administration (FDA) has also said that the drug might prevent a fertilized egg from attaching to the uterus for an outcome some consider equal to abortion.
Medical authorities have called this understanding outdated and scientifically incorrect, and cite clinical studies showing Plan B has no effect on fertilized eggs before or after implantation to the womb.
The debate grew louder when the Obama administration released its birth control coverage rules under the healthcare law.
The new policy requires most employers to cover a range of birth-control methods, including the so-called "morning-after pill," in their health plans without cost-sharing.
More than 100 plaintiffs have now filed suit to block the mandate on the grounds of their religious objections to abortion.
Murray is a staunch supporter of the Obama policy.
"Improving access to birth control is good health policy and good economic policy. It will mean healthier women, healthier children and healthier families. It will save money for businesses and consumers," Murray wrote in a February op-ed with Sens. Barbara Boxer (D-Calif.) and Jeanne Shaheen (D-N.H.).