Pershing Square Capital Management Chief Executive Officer Bill Ackman said he took down a critical post about UnitedHealth Group Inc. after the company’s lawyers told him the claims about the company that he amplified were untrue.

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.