The S&P 500 fell 0.83% to 1,408 in midday trading Thursday.
The Dow Jones Industrial Average was down 0.80% to 13,009, while the Nasdaq was off 0.83% to 2,965.
The yield on a 10-year treasury bond fell 2.67% to 1.71%.
The price of gold rose 0.17% to $1,664 per ounce.
Shares of Anaheim drug maker Questcor Pharmaceuticals Inc. fell about 7.5% to $28, amid a general decline among drug makers in afternoon trading Thursday. Questcor had a recent market value of $1.6 billion.
Shares of Seal Beach-based Clean Energy Fuels Corp. were off about 4% to $12 for a market value of $1.1 billion. Energy companies were generally mixed on Thursday.