Updated from 3:02 p.m. EST

Pharmaceutical firms

Pfizer

(PFE) - Get Report

and

Pharmacia

(PHA)

fell Thursday following a report that blockbuster arthritis drug Celebrex might be worse than expected at protecting the stomach from bleeding ulcers.

A study published in the

New England Journal of Medicine

said that contrary to some perceptions, patients at high risk for recurring bleeding ulcers taking Celebrex would be about as safe taking an older drug plus Prilosec. The study also found similar rates for kidney complications for both Celebrex and a combination of the older compound and Prilosec.

Pharmacia was down $2.02, or 4.7%, to $40.98, while Pfizer was off $1.36, or 4.3%, to $30.02. Pfizer announced in July it was acquiring Pharmacia in a $61 billion deal.

Celebrex and

Merck's

(MRK) - Get Report

Vioxx are newer arthritis compounds sold with the idea that they will ease arthritis pain with fewer gastrointestinal upsets than older compounds. Merck closed up a penny at $56.94.

A Merrill Lynch analyst downplayed the study's findings.

"Since most physicians already know that COX-II (coxib) inhibitors are not perfect and that there is still a risk for bleeding, we are not changing our

Celebrex

,

Vioxx

, or

Bextra

revenue estimates," said Merrill's David Risinger.