Boston Beer Co. Inc. lost about a quarter of its value after it said demand for Truly hard seltzers was lower than anticipated.

“We overestimated the growth of the hard-seltzer category in the second quarter and the demand for Truly, which negatively impacted our volume and earnings for the quarter and our estimates for the remainder of the year,” Chief Executive David Burwick said on a conference call Thursday. “Our outlook for the hard-seltzer category in the second half of 2021 is uncertain”

Shares of the company, which also sells Samuel Adams beer, fell about 24% to $717.17 at midday Friday.

The company increased its production of Truly for its summer peak but ended up with higher-than-planned inventory at its breweries and increased supply-chain costs, Mr. Burwick said.

The Boston-based company on Thursday missed Wall Street analysts’ earnings and sales projections for the fiscal second quarter. Boston Beer reported net income of $59.2 million for the three months ended June 26, slightly lower than the year-ago period as higher operating expenses eclipsed revenue growth. The company posted growth in depletions, or sales by distributors to retailers, of 24% for the second quarter, but the metric slowed from its first-quarter growth of 48%.

A maturing market and new hard-seltzer brands that “resulted in a proliferation and consumer confusion” contributed to the company’s challenge in growing its hard-seltzer category, founder and Chairman C. James Koch said on the conference call.

Mr. Koch said the company faced a tough comparison for the quarter, as the same period a year earlier saw customers loading their pantries amid Covid-19 restrictions.

Mr. Burwick added that certain canned products, such as its Twisted Tea brand, have gone out of stock. “We expect wholesaler inventories of Twisted Tea to remain tight for the rest of the summer,” he said.

The company’s finance chief said Twisted Tea has significantly lower-than-planned distributor inventory for certain styles and packages.

Boston Beer cut its forecast for the year. It said it expects increases in shipments and depletions of 25% to 40%, down from its prior estimate of 40% to 50%. The company plans to spend $80 million to $100 million on advertising, promotional and selling activities, down from $130 million to $150 million it guided previously. For the year, the company said it expects national price increases of 1% to 3%.