NEW YORK (TheStreet) -- Stocks made a dramatic downward turn Friday in the closing hour of trading as volatile, high-momentum health care stocks fell sharply in a continuation of trouble that began earlier in the week. 

Biotech stocks closed out their worst week in four years after Democratic presidential candidate Hillary Clinton vowed on Tuesday to address "price gouging" in the industry following the price increase of parasitic infection drug Daraprim from $13.50 to $750 a dose.

All benchmark indexes had been sharply higher earlier in the day, buoyed by clearer direction from the Federal Reserve Chair Janet Yellen on monetary policy. By the end of the session, the S&P 500 was flat, and the Nasdaq slid 0.9%. The Dow Jones Industrial Average remained up 0.77%, boosted by Goldman Sachs (GS) - Get Report and JPMorgan (JPM) - Get Report .

On Friday, Gilead Sciences (GILD) - Get Report fell more than 2%, Valeant Pharmaceuticals (VRX) dropped 4.9%, Celgene (CELG) - Get Report fell 4.7%, and Pfizer (PFE) - Get Report slumped 0.8%. The iShares NASDAQ Biotechnology Index ETF (IBB) - Get Report slid 4.9%, while the Health Care SPDR ETF (XLV) - Get Report tripped 2.7%.

In a speech at the University of Massachusetts on Thursday, Yellen made the case for a rate hike this year, though argued that the decision would continue to rely on economic data. Yellen said that inflation would build gradually over the next few years.

"Most FOMC participants, including myself, currently anticipate that achieving these conditions will likely entail an initial increase in the federal funds rate later this year, followed by a gradual pace of tightening thereafter," Yellen said.

Markets have been erratic since the Fed decided to leave rates unchanged at their meeting last week with investors interpreting that as another sign of uncertainty.

"The reason why the stock market traded off after [the Fed's decision to hold rates] was it created a lot of doubt in the minds of investors about the health of the U.S. economy," Don Ellenberger, senior vice president at Federated Investors, told TheStreet. Yellen "created a lot of uncertainty about what the Fed's strategy was for when they would begin raising rates."

Crude oil closed higher after the number of active drilling rigs in the U.S. dropped for their fourth straight week. A weekly rig count fell by 4 to 838, according to Baker Hughes data. West Texas Intermediate added 1.8% to $45.64 a barrel.

The final read on consumer sentiment in September moved higher to 87.2 from an initial reading of 85.7, according to the University of Michigan. The measure was above economists' estimates, though well below a reading of 91.9 in August.

The U.S. economy grew at a 3.9% annual pace in the second quarter, the Commerce Department said Friday. The GDP estimate was the second revision, up from a prior estimate of 3.7%. Consumer spending rose 3.6% from an initial estimate of 3.1%.

Google (GOOGL) - Get Report fell more than 2% after the Federal Trade Commission began an investigation into complaints the search engine uses its Android operating system to promote its own services such as Google Maps, according to The New York Times.

Volkswagen (VLKAY) is expected to announce its new CEO on Friday. CEO Martin Winterkorn, who resigned on Wednesday, will reportedly be replaced by Porsche's CEO Matthias Muller in the wake of a massive emissions scandal, according to The Wall Street Journal. Earlier this week, the German automaker admitted that more than 11 million diesel vehicles had software that gamed emissions tests. 

Marvell Technology (MRVL) - Get Report added 3.4% after announcing plans to cut around 17% of its work force. The layoffs are expected to save between $170 million and $200 million annually.

In earnings news, BlackBerry (BBRY) fell more than 7% after another weak quarter. The company reported a net loss of 13 cents a share, 4 cents wider than expected, while revenue tumbled 47% to $490 million.

Pier 1 Imports (PIR) - Get Report fell nearly 13% after quarterly profit missed estimates and full-year guidance was slashed. The retailer expects full-year profit as high as 64 cents a share, below previous guidance of 83 cents to 87 cents a share.

Bed, Bath & Beyond (BBBY) - Get Report shares were on watch after a weak quarter. Same-store sales rose 0.7%, or 1.1% on a constant-currency basis, well below estimates for 2% to 3% growth. The retailer also announced a $2.5 billion share buyback program.

Nike (NKE) - Get Report added 8.9% after reported net income of $1.34 a share, 15 cents above estimates, in its first quarter. Sales rose 14% on a constant-currency basis, while future orders jumped 9%. By brand, Nike sales gained 15% and Converse revenue climbed 3%.