Pfizer Inc.'s little blue wonder pills aren't being prescribed as much as some Wall Street pros had anticipated.

Morgan Stanley Dean Witter lowered its investment rating and its earnings expectation for the drug giant Friday over concern about prescription sales of the company's treatment for impotence, Viagra, which have fallen short of the firm's expectations.Shares of Pfizer Friday fell $4.50, or 4.5 percent, to close at $95.371/2 on the New York Stock Exchange.

Morgan Stanley downgraded Pfizer stock to neutral from outperform. Morgan Stanley also shaved its earnings outlook for Pfizer. Analyst Paul Brooke cut his 1998 earnings estimates to $2.11 a share from $2.15, and his 1999 estimates to $2.60 a share from $2.70.

Brooke reduced his 1998 global sales forecast for Viagra to $836 million from $1 billion.

New prescription levels of the blockbuster impotence drug peaked in the week ended May 8 at 279,000 and have "fallen consistently" to 85,000 in the week ended Aug. 21, according to the Morgan Stanley report. Refills of the drug have flattened out, the note said.

A Pfizer spokesman said the company doesn't comment on an-a-lyst reports.

At least three analysts who rate the company a buy said they are maintaining their ratings.

Since its launch, Viagra prescriptions have reached about 4 million, a Pfizer spokesman said.

The Food and Drug Administration confirmed late last month that 69 Americans have died after taking Viagra, but the agency hasn't changed its opinion about the drug's safety.

NationsBanc Montgomery Securities analyst Leonard Yaffe, who also rates Pfizer a buy, said the company remains Nations-Banc's favorite drug stock.

"We think that Viagra will remain the drug of choice for male erectile dsyfunction," he said.