WASHINGTON -- Merck & Co., the world's No. 3 drugmaker, won Food and Drug Administration approval yesterday for its once-a-day painkiller Vioxx, a rival to Monsanto Co.'s Celebrex in the $8 billion-a-year arthritis painkiller market.

Merck said Vioxx will arrive in U.S. pharmacies by mid-June.

Vioxx will compete directly with Celebrex, which has become the second-most-prescribed arthritis drug after entering the market in January. Developed by Monsanto, Celebrex is co-marketed by Pfizer Inc.

Merck needs a new winning product to offset an expected decline in revenue after 2001, when four of its drugs -- with combined annual sales of about $5 billion -- will lose patent protection. Analysts said Vioxx and Celebrex each have sales potential of more than $2 billion annually.

"Vioxx is a lot more important to Merck than they like to tell you," said Erik Anderson, a portfolio manager and analyst with Sit Investment Associates, which holds about 1.1 million Merck shares. "The bottom line is, they really need Vioxx to work."

Both Vioxx and Celebrex are designed for adults with acute pain, especially arthritis and menstrual pain, who can't tolerate prescription-strength painkillers such as ibuprofen. More than 3.4 million prescriptions have been written for Celebrex since January.

Merck shares fell 93.75 cents yesterday, to close at $71.3125.