Investors sold off shares of pharmacy giant CVS Health and Walgreens on Wednesday amid renewed speculation that Amazon would enter the business of selling prescription drugs.

As the stock market suffered its biggest loss since before the November election, shares of CVS and Walgreens closed down more than 3 percent, to $76.34 and $81.73, respectively.

CNBC reported Tuesday that the online retail giant is considering wading into the complicated retail pharmacy market. The report said Amazon is hiring a general manager to lead a team to explore the idea, but hasn’t made a final decision.

An Amazon spokeswoman did not immediately respond to a request for comment. (Amazon chief executive Jeffrey P. Bezos owns The Washington Post.)

Selling prescription drugs is a big business in the United States – retail pharmacy is a $400 billion industry, according to an analysis by Adam Fein, president of Pembroke Consulting. The top 15 pharmacies – including CVS, Walgreens, Express Scripts and Wal-Mart – accounted for about three-fourths of all prescriptions by revenue in 2016, Fein said.

Fein said he could see Amazon most easily entering the market for patients who pay cash, such as for generic drugs or brand-name drugs with discount coupons from manufacturers. Those patients may choose to pay for the drugs themselves because they’re uninsured or they’re looking for a better deal than what they would get through their insurer, which could force them to pay the full list price before they hit their deductible.

The pharmaceutical industry has been criticized for lacking transparency. Drug list prices do not reflect what most people eventually pay for the medicines, and secret rebates and discounts flow between various middlemen in the industry. Many patients who go to fill a prescription have little idea what price they’ll be charged at the pharmacy counter or whether they could get that drug cheaper elsewhere.

“Having a new competitor in the market would be great. Having a competitor that would potentially be transparent would be potentially even more interesting,” said Kevin Schulman, a professor of medicine at Duke University who has studied the web of financial relationships between drug companies and patients.