Shares of Medifast Inc., which sells weight-loss meals and counseling, dropped 13 percent, the most in almost two years, to $25.50, after its acting chief financial officer, Edward J. Powers, said he would resign, six weeks after the company’s prior CFO stepped down. Powers will leave no later than Jan. 4.

The CFO turnover and attention on weight-loss company Herbalife unnerved Medifast investors, said Kurt Frederick, a Wedbush Securities analyst. Medifast is “set up very differently from Herbalife,” he added.