The US Food and Drug Administration (FDA) has approved over-the-counter sales of the emergency contraceptive pill after three years of controversy.
Plan B will be on sale without a prescription by the end of the year
The drug, Plan B, will be sold only to women over the age of 18. Younger people will still need a prescription.
The makers, Barr Pharmaceuticals, say Plan B will be sold at pharmacies but not at corner shops or petrol stations.
The pills may prevent pregnancy if a woman takes them within 72 hours of having sexual intercourse.
They work by stopping or delaying ovulation, or by stopping an egg settling in the womb.
Barr has said it hopes to begin non-prescription sales of Plan B by the end of the year.
"While we still feel that Plan B should be available to a broader age group without a prescription, we are pleased that the agency has determined that Plan B is safe and effective for use by those 18 years of age and older as an over-the-counter product," said company chief executive Bruce Downey.
The pills will be kept behind pharmacy counters and the drug manufacturers will send anonymous shoppers to check whether pharmacists are enforcing the age restriction.
While women's rights groups claim the sales approval could halve the number of unplanned pregnancies in the US, conservatives warn the decision could encourage sexual promiscuity.
Pharmacists in the UK have been allowed to sell the morning-after pill without a prescription since 2001.