Investors began backpedaling from AMC Entertainment Holdings Inc. after the movie-theater operator said it plans to sell shares—even as it cautioned buyers that they may lose all their money—a day after the meme stock almost doubled in value.

Shares of AMC recovered some ground after dropping more than 25% in morning trading, recently trading down 4.7%. The moves erased some of the big gains from earlier in the morning before markets opened.

The company filed with regulators to sell more than 11 million shares, and warned against investing in its Class A stock. In a filing, the company said, “Our current market prices reflect market and trading dynamics unrelated to our underlying business.”

AMC shares have rocketed in recent days—extending their advance this year to 2,850% by Wednesday’s close—after the company sold a chunk of new shares to hedge fund Mudrick Capital Management LP. The company had leaned into its sudden popularity with individual investors by offering popcorn to shareholders who attend an AMC cinema this summer.

The firm appeared to backtrack on that stance Thursday. In a filing with the Securities and Exchange Commission detailing an agreement to sell shares, AMC said its share price may continue to experience extreme volatility that could cause losses for investors who buy the stock.

AMC’s filing, laying out plans to sell stock while saying investors who buy it are taking a big risk, was reminiscent of Hertz Global Holding Inc.’s plan to sell up to $500 million in shares last summer. In bankruptcy at the time, the car-rental company hoped to capitalize on strong demand from individual investors while warning that the bankruptcy process could wipe out shareholders.

Hertz suspended the sale after the SEC said it had questions about the deal, but the firm is emerging from chapter 11 after institutional investors won a contest to buy it out of bankruptcy. Individual investors who flocked to the company when it was in distress last year, not heeding the warnings of finance professionals, are poised to make big gains.

Despite the rockiness in AMC’s shares, the influence of online traders on the stock market showed no sign of abating. Shares of BlackBerry Ltd. , Sundial Growers Inc. and other meme stocks gyrated Thursday.

Individual traders have sent jitters through Wall Street since January when they fueled a buying spree in shares of small, often struggling companies such as GameStop Corp. and AMC. The frenzy has reignited in recent days, surprising some professional investors who had expected the sway of amateur traders bartering tips, wins and losses online on the market to wane.

AMC’s recent leap likely was amplified by the significant amount of stock that had been sold short by investors seeking to benefit from a decline in the company’s share price. When prices start to rise, inflicting losses on short sellers, they often buy back borrowed shares they had sold, fueling further gains.

Of AMC’s publicly traded shares, about 18% had been sold short Wednesday, according to S3 Partners. Investors with short positions in AMC have been hit with roughly $5.2 billion in realized and unrealized losses this year, S3 data show, including nearly $2.8 billion on Wednesday alone when the stock price jumped 95% in one day.

Amid another frenetic trading session, shares in BlackBerry rose 4%. They had earlier jumped over 16% and added to their 32% rally Wednesday as individual investors put out calls online to take the price “to the moon.”

The move in BlackBerry shares had no obvious catalyst, instead appearing to be propelled by a fresh burst of interest in the stock on internet groups including Reddit’s WallStreetBets. More than 2,300 comments about the company were posted on the forum before the bell Thursday, according to data from TopStonks.com, outstripping mentions of AMC.

After sagging for most of the past decade, BlackBerry shares had jumped 130% this year to close at $15.25 apiece Wednesday. That compared with the S&P 500’s 12% advance this year. The provider of security software and services reported a net loss of $1.1 billion on revenue of $893 million in the year through February.

Meme stocks hold growing influence in the broader stock market thanks to their soaring market values, which many investors view as a way to make quick money by riding their upward momentum. In an unusual twist, they have also attracted value investors, who typically look for bargains in the stock market and bet that their prices will appreciate.

GameStop and AMC can now be found in a range of exchange-traded funds, from value funds to growth funds and social-media-focused funds.

AMC said it would sell the stock—valued at about $722 million based on Wednesday’s closing price of $62.55 a share—from time to time via its banks, B. Riley Securities Inc. and Citigroup Inc. The proceeds could go toward paying down its debt or buying movie theaters, the cinema chain said.

AMC struggled when cinemas closed during the pandemic and said as recently as December that it could go bankrupt. In January the company said it had raised $917 million in financing to ward off bankruptcy and AMC has now taken advantage of a soaring share price to raise cash twice within a week.

—Caitlin Ostroff and Caitlin McCabe contributed to this article.

Write to Joe Wallace at Joe.Wallace@wsj.com