Shares of Weight Watchers International surged as much as 26 percent on Wednesday after media guru Oprah Winfrey said she lost more than 42 pounds on the program.
The stock ended the day nearly 21 percent higher. The company has a market cap of about $855.9 million,
On Tuesday, Winfrey said on CBS' "The Late Show with Stephen Colbert" that she was down 42.5 pounds on Weight Watchers.
"Everybody who has watched me over the years knows I've struggled with food. It's the first time I've actually felt that I've gotten it under control," she said on Colbert's show.
The weight-loss company's stock experienced a similar rise in December when Winfrey announced she lost 40 pounds on the program, alongside the introduction of the company's "Live Fully" campaign.
Winfrey and Weight Watchers announced a partnership in October 2015, causing the company's share price to instantly double. Months later, she said she had lost 12 pounds on the diet. Winfrey has a 9.97 percent stake in the company, according to FactSet.
Sidoti analyst Frank Camma upgraded the firm's price target for the company to $28 from $25, citing strong macro factors and a refreshed Oprah marketing campaign.
With Wednesday's gains, Weight Watchers's stock has climbed 14 percent in the last six months.