Stocks fell Friday in heavy trading as drug giant Pfizer disclosed potentially devastating links between its blockbuster medication Celebrex and heart risks, renewing investors' concerns over the pharmaceutical sector. The three major indexes finished the week higher.

The Dow Jones industrial average (search) ended down 55.72 points, or 0.52 percent, at 10,649.92. The Standard & Poor's 500 Index (search) closed down 8.99 points, or 0.75 percent, at 1,194.22. The technology-laced Nasdaq Composite Index (search) was down 10.95 points, or 0.51 percent, at 2,135.20.

For the week, the Dow gained 106.70, or 1.01 percent; the S&P was up 6.20, or 0.52 percent; and the Nasdaq climbed 7.13, or 0.34 percent. Stocks have been up six of the last eight weeks.

"Pfizer was clearly what weighed on the market, along with crude strengthening to levels where we should show concern again," said Dan McMahon, head of listed trading at CIBC World Markets Inc.

Pfizer Inc. (PFE) pulled the Dow lower at the start of the session and kept pressure on the index after it said patients taking Celebrex in a long-term cancer-prevention trial had more than twice the number of fatal or non-fatal heart attacks as those taking a placebo. A similar revelation led to the removal of Merck & Co.'s (MRK) competing Vioxx drug earlier this year — and led to a major selloff of Merck shares. Pfizer said it will leave Celebrex on the market.

Meanwhile, Eli Lilly and Co. (LLY) announced it was adding a warning to the label of Strattera, its medicine to treat attention deficit and hyperactivity disorder, advising patients with jaundice or a liver injury to stop taking the treatment.

Despite moderate losses during the session, analysts remained bullish on the overall market.

"The nice thing about this market is that you may get a selloff of Pfizer, but the rest of the market behaves incredibly well," said Mark Bronzo, managing director of Gartmore Separate Accounts LLC. "The market's acting like people want to be in it. So when you get something like this, people find other places to put their money and don't just sell everything."

Pfizer shares slid 11.1 percent to $25.75, after earlier falling as low as $23.52, the lowest it's been in about seven years. Eli Lilly slipped 2.4 percent to $56.02, after falling as low as $53.90 earlier in the session.

In other drug sector news, Britain's AstraZeneca (AZN) said its lung-cancer drug Iressa did not help patients live longer. American Depositary Receipts of AstraZeneca closed down 7.7 percent at $37.10.

The American Stock Exchange Pharmaceutical Index fell 3 percent, with 13 of its 15 components ending lower.

In addition to the drug sector, a number of technical events affected the market, quadruple witching being among them, said Steven Schoenfeld, chief investment strategist at Northern Trust Global Investments.

Quadruple witching is a term used by pros to describe the quarterly expiration of four different types of options and futures. The event can cause some added volume and volatility as investors offset or close out their positions at the last minute.

Stocks also saw pressure from crude oil futures, which shot past $46 per barrel on concerns about higher demand due to a colder-than-expected winter. A barrel of light crude settled at $46.28, up $2.10, on the New York Mercantile Exchange (search).

The pharmaceutical sector has been under intense pressure since Merck's Sept. 30 announcement that it would pull Vioxx from the market due to increased risk of heart attack and stroke found among its users. At the time, Pfizer maintaned Celebrex was safe, but investors began to worry that hidden health problems, whether overlooked by drug companies or the U.S. Food and Drug Administration (search), could derail any number of top-selling prescription drugs.

"Drug stocks are growth stocks without any growth prospects," said Russ Koesterich, U.S. equity strategist at State Street Corp. in Boston. "You look at the pipeline of future products — there are no pipelines. They're coming under pricing pressure from the government. There's generic competition. Why buy the stocks?"

In the retail sector, watched closely for signs of a holiday shopping turnaround, Circuit City Stores Inc. (CC) slipped 56 cents to $14.72 after reporting a narrower-than-expected loss for the quarter. The company lost 3 cents per share, while Wall Street expected a loss of 8 cents per share. Revenues, however, were slightly below analysts' forecasts.

Profits at sporting goods giant Nike Inc. (NKE) rose 47 percent for the second quarter, with a weak dollar helping overseas sales. The company beat Wall Street forecasts by 11 cents per share. Nike was up $5.80 to $91.70.

Pfizer's announcement drew attention away from the Labor Department's (search) consumer price index (search) report for November. The CPI rose 0.2 percent in November, coming off an 0.6 percent rise in October, while "core" CPI — excluding food and fuel costs, which vary greatly — also rose 0.2 percent for November. The results were in line with Wall Street's forecasts, and show that inflation isn't a major factor in the economy.

With trades still settling late in the day, Friday's session was the busiest day of the year and fifth busiest day of all time, according to statistics on the New York Stock Exchange's Web site.

Trading was heavy, with nearly 2.5 billion shares changing hands on the New York Stock Exchange, well above the 1.4 billion daily average for last year. About 2.4 billion shares were traded on Nasdaq, above the 1.69 billion daily average last year.

The ratio of advancing stocks to declining stocks was about even on the Big Board and the Nasdaq.

The Russell 2000 index of smaller companies was down 0.15, or 0.02 percent, at 642.08.

Overseas, Japan's Nikkei stock average rose 1.41 percent. In Europe, Britain's FTSE 100 was down 0.81 percent, France's CAC-40 tumbled 1.73 percent, and Germany's DAX index fell 1.22 percent.

Reuters and the Associated Press contributed to this report.