Bangladesh, Nov. 17 -- Now at Amazon.com: insulin and inhalers.

The online colossus opened an online pharmacy Tuesday that allows customers to order medication or prescription refills, and have them delivered to their front door in a couple of 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 9%. Walgreens and Rite Aid both tumbled more than 10%.

The big chains rely on their pharmacies for a steady flow of shoppers who may also grab a snack, or shampoo or groceries on the way out. All have upped online services, but Amazon.com has mastered it, and its online store is infinitely larger.

Amazon will begin off...