Wyeth, Pfizer Fall on Downgrades  NEW YORK (Reuters) - Shares of Wyeth &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=WYE.N target=/stocks/quickinfo/fullquote"&gt;WYE.N&lt;/A&gt; and Pfizer  Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=PFE.N target=/stocks/quickinfo/fullquote"&gt;PFE.N&lt;/A&gt; fell on Monday after Merrill Lynch downgraded the  drugmakers and U.K. regulators issued tougher warnings on the  type of popular depression drugs sold by the companies.