NEW YORK (CNNMoney.com) - Lipitor, the world's top-selling drug from the world's biggest drug company, created cause for concern in Pfizer's 2005 fourth quarter.

Pfizer triumphantly announced Thursday that worldwide sales for Lipitor, a cholesterol-reducing statin, surged 12 percent in 2005 to $12.2 billion, a record high for annual revenue from any drug. Pfizer (up $0.86 to $24.95, Research) stock was up almost 4% in afternoon trading on the New York Stock Exchange.

However, analysts tuning into Pfizer's earnings webcast this afternoon were more concerned about where Lipitor sales are going in 2006. In the fourth quarter, 2005, Lipitor sales grew 3 percent from the fourth quarter, 2004, arousing concerns among analysts that sales growth is slowing.

Peter Brandt, senior vice president for Pfizer Global Pharmaceuticals Finance, told analysts "it's too early to tell whether we have a trendable event" in the slowing of Lipitor sales.

Brandt and chief executive officer Henry McKinnell also said that Lipitor sales could benefit from coverage in the newly enacted Medicare prescription drug benefit and from increased popularity in Germany. However, Brandt and McKinnell declined to provide more detail, saying that such questions will be answered at an investor's conference on Feb. 10.

"The Medicare prescription drug benefit is certainly a positive," said Brandt. "That's something, as Hank said, that we hope to have a better handle on sometime in February."

Pfizer also plans to release financial forecasts at its Feb. 10 meeting. Last October, Pfizer pulled guidance for 2006 and 2007, triggering a loss of about one-quarter of its value. Pfizer's stock has since recovered from the financial guidance fallout.

In fourth quarter, 2005 earnings announced today, Pfizer said quarterly sales for the company fell 9 percent to $13.6 billion. Net income also slipped in the fourth quarter, to $2.7 billion, or 37 cents per share, from $2.8 billion, or 38 cents, in the same period the year before.

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To read more about Pfizer's fourth quarter earnings, click here.