The results, and the company's full-year outlook, were short of Wall Street analysts' expectations.
The New York company earned 12 cents a share compared with 20 cents in the year-earlier quarter.
The latest earnings reflect charges of 31 cents a share to extinguish debt early in connection with a refinancing and 5 cents a share tied to restructuring.
Shares outstanding rose 1.9% to 71.2 million.
Revenue fell to $311.4 million from $333.6 million.
WW shares on Tuesday were down 21% to $25.42 in after-hours trading at last check. The stock had closed up 4.3% to $32.29.
A survey of analysts by FactSet was expecting GAAP profit of 68 cents a share, or an adjusted profit of 66 cents a share, on revenue of $337.5 million.
"The strong digital year-over-year growth momentum in the first quarter slowed in the second quarter as we cycled against strong digital performance in 2020," President and Chief Executive Mindy Grossman said in a statement.
WW ended the quarter with 4.9 million subscribers, including 4.1 million digital subscribers. The total was up 6% year-over-year but below the company's expectations, Grossman said.
"We have a comprehensive plan to optimize performance in the second half of the year and position us well for growth in 2022," she said.
For the full fiscal year, WW estimates GAAP earnings at $1.10 to $1.25 a share, including debt-related and restructuring charges of about 53 cents a share. Revenue should approach $1.3 billion.
For the year, the FactSet survey was calling for GAAP earnings of $2.19, or an adjusted $2.08, on revenue of $1.39 billion.