WASHINGTON - Pfizer Inc won U.S. and Canadian antitrust approval to buy smaller rival Wyeth on Wednesday, and will close the $66 billion deal this week, the world's largest drugmaker said.

The deal is designed to soften the blow when Pfizer loses U.S. patent protection for its $11 billion a year cholesterol medicine Lipitor in 2011 by adding Wyeth's lucrative vaccines and injectable biologic medicines.

Pfizer said it expects to complete the deal on Thursday after getting clearance from both Canadian and U.S. antitrust regulators on Wednesday.

Wyeth shares were up 1.5 percent to $49.88 in afternoon trading, while Pfizer shares were up 2.4 percent to $17.19, both on the New York Stock Exchange.

The U.S. Federal Trade Commission said the potential harm of the deal, as originally structured, was confined to animal health products.

The commission concluded that the transaction does not raise anticompetitive concerns in any human health product markets, the FTC said in a statement.

To win U.S. approval, Pfizer agreed to sell about half of Wyeth's Fort Dodge U.S. animal health business to Boehringer Ingelheim Vetmedica Inc. Pfizer will also sell its horse vaccines to Boehringer Ingelheim.

The terms of that deal, which was announced on Sept. 21, were not disclosed.

Pfizer had previously won approvals from antitrust enforcers in the European Union, China and Australia.

The Wyeth deal marks the third giant acquisition for Pfizer in the last nine years. It bought Warner-Lambert for $114 billion in 2000, gaining full ownership of Lipitor, and acquired Pharmacia in 2003 for $60 billion. (Reporting by Diane Bartz; additional reporting by Bill Berkrot in New York; editing by Andre Grenon, Tim Dobbyn, Dave Zimmerman)