The Dow slipped into negative territory while the S&P 500 and the Nasdaq pared gains on Thursday, as drugmaker Merck fell on concerns over its cholesterol drug.

The Dow Jones industrial average <.DJI> dropped 7.94 points, or 0.10 percent, to 8,170.47. The Standard & Poor's 500 Index <.SPX> gained 1.84 points, or 0.21 percent, to 881.40. The Nasdaq Composite Index <.IXIC> added 6.26 points, or 0.36 percent, to 1,753.43.

(Reporting by Chuck Mikolajczak; Editing by Leslie Adler)