Stocks Knocked Lower by Merck Drug Recall  NEW YORK (Reuters) - The blue-chip Dow Jones average fell  on Thursday after drug maker Merck   Co.&lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt; withdrew its  widely used Vioxx painkiller after evidence showed the drug  could increase risk of heart attack and stroke.