Merck Pulls Arthritis Drug from 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; pulled its  arthritis drug Vioxx off the market on Thursday after a study  showed it doubled the risk of heart attack and stroke. The move  sent the company's shares plunging almost 27 percent and erased  \$25 billion of its market value.