SEATTLE — Amazon has agreed to buy the Boston-based online pharmacy PillPack. Terms of the deal were not disclosed.

Shares of CVS Health Corp. and Walgreens Boots Alliance were trading lower Thursday morning following the announcement of the deal, which marks a big move by Amazon into the pharmacy business. PillPack is already licensed to sell mail order prescriptions in 49 states. The company is also is an in-network pharmacy with all the major pharmacy benefit managers, including the following: CVS Caremark, Express Scripts, Optum Rx, Prime Therapeutics, Humana Pharmacy Solutions, Cigna, Aetna, MedImpact, EnvisionRx, and CastiaRX. As part of these relationships, PillPack is also in network with most major Medicare Part D plans.

PillPack  bills itself as offering the best possible customer experience in the U.S. for people who take multiple daily prescriptions. The company said it delivers medications in pre-sorted dose packaging, coordinates refills and renewals, and makes sure shipments are sent on time.

“PillPack’s visionary team has a combination of deep pharmacy experience and a focus on technology,” said Jeff Wilke, Amazon chief executive officer worldwide consumer. “PillPack is meaningfully improving its customers’ lives, and we want to help them continue making it easy for people to save time, simplify their lives, and feel healthier. We’re excited to see what we can do together on behalf of customers over time.”

PillPack was founded in 2014 by T.J. Parker, a second generation pharmacist, and Elliot Cohen, an engineer.

“PillPack makes it simple for any customer to take the right medication at the right time, and feel healthier,” said Parker, who is chief executive officer of PillPack. “Together with Amazon, we are eager to continue working with partners across the healthcare industry to help people throughout the U.S. who can benefit from a better pharmacy experience.”

Completion of this transaction is subject to regulatory approvals and other customary closing conditions. The parties expect to close the deal during the second half of 2018.