UnitedHealth shares fell more than 4% on Wednesday before finishing down 1%, a day after hedge-fund manager Bill Ackman wrote a post on social media that said the insurer could be overstating its profits because it denies patients medically necessary care.

UnitedHealth Group has raised concerns with the top US securities regulator over a social media post by activist investor Bill Ackman, who claimed that the healthcare group could be inflating its profits.