Pfizer (NYSE: PFE) will soon get a sneak preview of cholesterol drug Lipitor's impending tumble from its top-selling pedestal. Yesterday, Health Canada approved a few generic versions of Lipitor, including ones from Teva Pharmaceutical (Nasdaq: TEVA), Watson Pharmaceuticals (NYSE: WPI), and privately held Apotex.

Losing Lipitor in the Great White North won't kill Pfizer. With sales of about $1.2 billion in Canada, the drugmaker stands to lose about 10% of Lipitor's sales. But Pfizer's acquisition of Wyeth last year made it less dependent on the drug. Those Canadian sales make up less than 2% of expected revenue for this year, and Pfizer had already factored the loss into its guidance.

U.S. sales could suffer if generic versions of Lipitor slip over the border, but only slightly. Cheaper brand-name drugs in Canada don't dent U.S. sales much.

The big loss of revenue will come in November 2011, when the drug begins to face generic competition in the U.S. Stateside sales topped $5.6 billion last year, making off-brand Lipitor a major bounty for generic-drug makers like Teva, Novartis (NYSE: NVS), and Mylan (Nasdaq: MYL).

Pfizer will try to retain revenue from some patients by launching its own generic through its established product division. But the revenue decline could still be severe; investors should be happy if the company can retain 20% to 30% of sales in a given country when Lipitor's patent protection expires.

Other cholesterol drugs, including Merck's (NYSE: MRK) Vytorin and AstraZeneca's (NYSE: AZN) Crestor, could also be hurt by the availability of generic Lipitor. Doctors may be more likely to prescribe a cheap generic version of Lipitor over a more expensive brand-name drug from another company. That indirect generic competition affected Lipitor sales when Merck's Zocor went generic a few years ago.

The only good thing about drugs gaining generic competition is that investors can see it coming; it's not as bad as a drug suddenly being taken off the market because of unforeseen side effects. Pfizer will survive the loss of Lipitor; I'm just not sure how much increased revenue from other drugs will be able to compensate for the loss.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Novartis is a Motley Fool Global Gains pick. The Fool has a disclosure policy.