profit warning by UnitedHealth (UNH.N) led to a 5.65 percent drop in its stock, making the health insurer the biggest drag on the Dow Jones industrial average and the S&P 500. It also sent the shares of competitors Anthem (ANTM.N) and Aetna (AET.N) down more than 6 percent each.
The S&P healthcare sector .SPXHC was the worst performer among the 10 major S&P sectors with a 1.63 percent decline.
Adding to the pain in healthcare, drugmaker Pfizer (PFE.N) fell 3.06 percent after reports that its talks to buy Allergan (AGN.N) and redomicile in Ireland were in final stages. Allergan lost 2.81 percent.