NEW YORK — Now at Amazon.com: insulin and inhalers.

The online colossus opened an online pharmacy today that allows customers to order medication or refills, and have them delivered to the front door in a couple days.

The potential impact of Amazon’s arrival in the pharmaceutical space rippled through that sector immediately. Before the opening bell, Shares of CVS Health Corp. fell almost 7%. Walgreens tumbled 10%.

The big chains rely on their pharmacies for a steady flow of shoppers that may also grab a snack, or shampoo or groceries on the way out.

Amazon will begin offering commonly prescribed medications today, including creams, pills, as well as medications that need to stay refrigerated, like insulin. Shoppers have to set up a profile on Amazon’s website and have their doctors send prescriptions there.

Most insurance is accepted, Amazon said. But Prime members who don’t have insurance can also buy generic or brand name drugs from Amazon for a discount.

Amazon has eyed the health care industry for some time. Two years ago, it spent $750 million to buy online pharmacy PillPack, which organizes medication in packets by what time and day they need to be taken. Amazon said that PillPack will continue, focusing on shipping medication to people with chronic conditions.