Viagra, Pfizer Inc.'s new treatment for impotence, could have the fastest takeoff, measured in prescriptions written, in drug history. As the only pill for impotence in a sexually active culture, Viagra sales could soar to several billion dollars annually in a year or two, securities analysts said Friday.

But some warned of possibly disappointing sales if many users have exaggerated expectations for the drug, possibly in trying to enhance recreational sex. These analysts said Viagra would be most effective for men with moderate impotence problems.Shares of Pfizer, based in New York, have surged 21 percent since Feb. 1, to a high of $97.50 on the New York Stock Exchange last Monday, in anticipation of Friday's approval of Viagra by the Food and Drug Administration. Pfizer closed at $95.75 Friday, up $1.375. At 47 times earnings, it was the priciest drug stock.

Neil Sweig, who follows Pfizer for Southeast Research Partners, said some analysts were saying "The company's name is no longer Pfizer, it's Viagra."

Pfizer said the wholesale price for Viagra would be $7 a pill, which analysts said would translate to $8.50 to $9 for consumers, making it affordable for most middle-class people. David Saks, an analyst at Gruntal & Co., projected Viagra sales could peak at $2.5 billion a year in the United States.

Other predictions put eventual worldwide sales as high as $4 billion, matching the current leading seller, Prilosec, a treatment for gastrointestinal ailments.

"There are many unknowns," Sweig said. "Will women use it? Will there be unknown, serious side effects if Viagra is abused for recreational sex or as an aphrodisiac, which it is not?"

Sweig projected $600 million in annual sales of Viagra by next year.

Steven Gerber, an analyst at CIBC Oppenheimer, said Pfizer would face other challenges. He said men had often been reluctant to try new drugs like Rogaine and Propecia for baldness and Proscar for enlarged prostates, none of which met analysts' optimistic projections. "Women are better at self-referral" for health problems, he said.

Furthermore, managed-care plans have tried to limit soaring drug spending by barring certain categories deemed not essential.