A CVS spokesman said that the company has implemented temporary purchase limits to ensure equitable access and that it has ample supply of the pills in stores and online. Rite Aid said it was limiting purchases due to increased demand. Walmart didn’t respond to requests for comment.

Walgreens Boots Alliance Inc., which also had a purchase limit on its website, said that the restriction was an error and that it would soon be corrected. A spokesman said the company is investigating the situation.

The pills are often referred to and sold under the Plan B brand without a prescription. Also called morning-after pills, they are designed to be taken up to three days after unprotected sex. The medication mainly works by preventing ovulation and, failing that, may stop a fertilized egg from attaching to the uterus.

Plan B pills are different from medication abortion, also known as plan C, which requires a prescription and involves the administration of different pills to terminate a pregnancy. In the U.S., medication abortion has been approved by the Food and Drug Administration for up to 10 weeks of pregnancy. Two medications—mifepristone and misoprostol—are typically used in a medication abortion regimen.