The day finally came when the world's largest drug company, Pfizer (PFE), lost patent protection on the best selling drug ever, cholesterol fighter Lipitor.

Lipitor has been a major source of income for the New York-based pharmaceutical. The pill was released in 1997, and by 2006 it had reached peak sales of $12.9 billion, accounting for 27% of the company's revenue. Even after Pfizer acquired Wyeth, Lipitor still accounted for 15.8% of total revenue in 2010, with $10.8 billion in sales. Altogether, Lipitor sales surpassed $100 billion.

But now it has lots more competition coming -- at dramatically cheaper prices. On Wednesday, the day Pfizer's decade and half-long patent expired, Watson Pharmaceuticals (WPI) launched an authorized generic version of Lipitor, atorvastatin. Pfizer isn't totally missing out. Part of its exclusive agreement includes paying Pfizer a share of its sales of generic Lipitor. Indian generics manufacturer, Ranbaxy Laboratories, will also market generic Lipitor. It won approval from the FDA Wednesday. Ranbaxy will share its first first six months' profit with Teva (TEVA).

After the six months exclusivity, other generic manufacturers will be able to enter the market as well, which would lower the price even more -- to about 80% of the branded version. That may cause Pfizer to lose 90% of the market, if it follows the path others have experienced.

Pfizer not giving up on Lipitor sales
But Pfizer, which was unable to offset the anticipated lost Lipitor sales in traditional ways, has devised other methods for keeping a big piece of the Lipitor pie for years to come.

Pfizer is providing discounts and incentives for patients and insurers among other practices, according to the AP. It also managed to get agreements with companies that process prescriptions to block the ability of the pharmacists to dispense the generic.

So while Lipitor sales are bound to decline, they could remain much larger than many had expected. In fact, Watson CEO told the AP and CNBC that he predicts Pfizer will keep 40%-45% of the market over the next six months. To wit, Watson shares closed nearly 4% lower Wednesday to $64.62. Pfizer shares, on the other hand, actually climbed 3.45% to close at $20.07. Throughout this week investors have cheered the maneuvers by the drugmaker.

So is it time to buy Pfizer?

Has investor fear of the Lipitor doomsday created a buying opportunity in the stock? This largely depends on your investment strategy and horizon.

Investors can focus on the fact that Pfizer was unable to properly prepare for the hit it's about to take to its revenue, either through research or though acquisition. But Pfizer does have several attractive features.

Just on Tuesday, the FDA accepted a Pfizer/Bristol-Myers Squibb (BMY) anti-clot drug Eliquis for fast review. The drug in question certainly has blockbuster potential. Another potential is Pfizer's rheumatoid arthritis drug tofacitinib. Others, such as the Prevnar vaccine, also hold great growth potential.

The company is also expected to spin off its animal health business. It gave no word yet on the plans for its infant formula business. The combined value of the businesses could reach about $25 billion to $32 billion, Reuters reported. And with its massive cash hoard, Pfizer can easily look elsewhere to complement its pipeline and compensate for R&D cuts.

Meanwhile, Pfizer also has one of the highest dividend yields among the Dow Jones stocks. At 80 cents a year, the yield is very near 4%.

When it reported third quarter results, the drugmaker raised its full-year earnings forecast. It also increased its 2011 share-buyback plan to between $7 billion and $9 billion, up from between $5 billion and $7 billion.

If investors expect a repeat of Lipitor, they shouldn't hold their breath. A drug on that magnitude is unlikely to come along again. As Mathew Herper of Forbes explains, the paradigm has shifted to more specialized medicines rather than a one-drug-fits-all. But Pfizer is not without its positives. And buying the stock in turbulent times could prove a good move for those with a long-term horizon.