US stocks edge lower ahead of Yellen speech

US stocks finished lower on Thursday ahead of a much-awaited speech by Federal Reserve Chair Janet Yellen that analysts hope will clarify the Fed's policy stance.

Health care and pharmaceutical shares took a hit as outrage mounted over alleged profiteering by Mylan NV, which has a near-monopoly position for its EpiPen epinephrine injectors and has jacked the price up five-fold to more than $600 over nine years.

At the close, the Dow Jones Industrial Average was down 0.2 percent at 18,448.41.

The broad-based S&P 500 slipped 0.1 percent to 2,172.47, while the tech-rich Nasdaq Composite was gave up 0.1 percent to 5,212.20.

Markets hope that after months of resisting an interest rate hike, Yellen will shed light on the Fed's view of US economic strength and the prospects of a possible rate rise in the coming months when she speaks at a central banking symposium at Jackson Hole, Wyoming at 1400 GMT Friday.

Yellen "should strike a moderately positive tone with respect to the near-term economic outlook," Deutsche Bank said in a client note.

"While we do not expect Yellen to explicitly pre-commit to raising interest rates at the September 20-21 (Fed) meeting, she will no doubt want to retain the optionality of hiking next month if economic and financial conditions permit."

After losing almost 11 percent over two days following attacks by top lawmakers, Mylan shed another 0.9 percent Thursday when its statement on how it would mitigate the burden of the EpiPen price on more needy customers fell flat.

Legislators and Democratic presidential candidate Hillary Clinton blasted the company for not straightforwardly cutting the price of EpiPen, which is used to counter extreme, life-threatening allergic reactions.

The tone of the criticism from lawmakers of both parties suggested increasing pressure against price increases from health care providers, insurers and drug makers.

Shares of the country's largest health care provider, UnitedHealth Group, dropped 1.8 percent, Cigna lost 1.5 percent and Aetna 1.0 percent.

US retailers turned in mixed quarterly results, with shares of large discount chains off sharply after disappointing earnings. Dollar General lost 17.8 percent and its rival Dollar Tree, 10.0 percent.

Sears Holdings beat analysts' forecasts handily with a much smaller quarterly loss than expected, but shares of the department store chain gave up 4.3 percent as its balance sheet showed a dangerously low level of cash and unsustainable debt.