Moody's Investors Service lowered its outlook to negative for the U.S. pharmaceutical industry, citing the loss of billions of dollars for companies such as Pfizer Inc. that face generic competition to top-selling drugs.

Patents on medicines that generate more than 40 percent of industry revenue will expire within five years, and the pace of new approvals by regulators is slowing, Moody's said in a statement today. The U.S. credit rating company, which evaluates 21 U.S. drugmakers with about $90 billion in rated debt and bank credit facilities, lowered its industry outlook from stable.

Pfizer, the world's largest drugmaker, stands to lose more than $20 billion after 2013 as products, including its top-selling cholesterol pill Lipitor, lose patent protection. At the same time, the number of new drugs approved between 1999 and 2006 fell 49 percent, according to data compiled by Bloomberg, and Moody's report cited "important new drug applications" that have been rejected or delayed.