text: Merck Pulls Arthritis Drug Off Market  NEW YORK (Reuters) - Merck   Co Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt; on Thursday  pulled its arthritis drug Vioxx off the market after a study  showed it doubled the risk of heart attack and stroke, a move  that sent its shares plunging and erased \$25 billion from its  market value.