Many market commentators think very highly of Pfizer (NYSE:PFE). Is this justified? Well, first off, it's one of the few companies with a AAA credit rating. It's obviously a mature and stable drug company. Further, the firm boasts a long list of drugs we have all heard of, and it devoted more than $7 billion to new drug research, which has allowed it to develop an enviable pipeline.

This has resulted in very solid fundamentals for the most part. Shares are a little expensive compared to the overall health-care group, trading at 5.5 times sales and 3.9 times book value. On the plus side, however, Pfizer has almost $14 billion in cash and returned $0.68 a year to shareholders in dividends (representing a 1.91% yield). Over the past five years, it has performed in line with the pharmaceutical index.

While Pfizer has done well, we should be more concerned about what the stock can do for us in the future. Unfortunately, the future may not be so great. Pfizer's revenue over the trailing 12 months was $49.15 billion. It is expected to grow sales this year to $52.83 billion and to $56.18 billion the next. That works out to growth rates of 7.4% and 6.3%, respectively. Is that the kind of growth rate you want to invest in?

The problem lies in its massive size. Drug companies hit home runs when they create billion-dollar medicines. Pfizer needs five blockbusters per year just to be mediocre. If it maintains a mid-single-digit growth rate, it will need seven new blockbusters to be mediocre five years from now. Uh-oh.

Pfizer had some negative news last week as a judge ruled that it will have to pay $430 million to settle an issue over the marketing of Neurontin. The judgment was for actions taken at Warner-Lambert's Parke-Davis division prior to Warner being acquired by Pfizer. This should not be a real problem for the company, though.

The point of this is not to besmirch Pfizer. Quite the contrary -- this is a great company. But it is not clear how the stock can outperform the market or even its sector for the foreseeable future.

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Fool contributor Roger Nusbaum is an investment manager and wildland firefighter in Prescott, Ariz. At press time, neither he nor his clients owned shares in Pfizer.