Anxiety over such a market upheaval is said to have partly driven CVS to enter into talks with Aetna, the large health insurer, over a potential acquisition. Larry Merlo, the president and chief executive of CVS Health, did not discuss the potential deal in a call with analysts on Monday.

Free delivery will be available “within hours” for prescription drug orders, as well as a “curated selection of over-the-counter products” in Manhattan beginning Dec. 4, the company said. Same-day delivery would expand to Miami, Boston, Philadelphia, Washington, D.C. and San Francisco early next year. Next-day delivery, from local stores, would become available in the rest of the country.

A CVS spokeswoman, Erin Pensa, said delivery fees outside of Manhattan will vary by location, and have not yet been made public. She also said the company’s delivery partner has not been announced, but “we’ve been able to use our scale to negotiate low-cost, affordable options for all CVS Pharmacy customers.”

“Our goal is to meet the needs of all of our customers wherever, however and whenever they want,” Helena Foulkes, executive vice president of CVS Health and president of CVS Pharmacy, said in a statement Monday.

CVS said Monday that retail sales, which include prescription sales, fell 2.7 percent in the third quarter, although that was offset by the performance of its pharmacy services business, which manages drug benefits for employers and insurers. That unit saw revenues increase by 8.1 percent. Overall, third-quarter revenues increased by 3.5 percent, or $1.6 billion, to $46.2 billion, compared to the third quarter of 2016.