Amazon.com Inc. is almost certain to enter the business of selling prescription drugs by 2019, said two analysts at Leerink Partners, posing a direct threat to the U.S.'s biggest brick-and-mortar drugstore chains.

"It's a matter of when, not if," Leerink Partners analyst David Larsen said in a report to clients late Thursday. "We expect an announcement within the next 1-2 years."

Amazon has a long standing interest in prescription drugs, an industry with multiple middlemen, long supply chains and opaque pricing. In the 1990s, it invested in startup Drugstore.com and Amazon Chief Executive Officer Jeff Bezos sat on the board. Walgreens eventually purchased the site and shuttered it last year to focus on its own branded website Walgreens.com.