Amgen (NASDAQ:AMGN) has been down a rocky road the last couple of years, with its anemia drugs getting bounced around by label changes, but it looks as though there's at least one more pothole in the road.

Plummeting sales of anemia treatment Aranesp seemed to be slowing, but the year-over-year change in the fourth quarter increased again.

2008

Q1

Q2

Q3

Q4

Year-over-year (decrease) in Aranesp sales

(25%)

(13%)

(3%)*

(15%)

Source: Company press releases.
*Returns adjustment of $54 million removed from year-over-year comparison.

Despite the lower sales of the former big seller, revenue for the quarter remained flat as anti-inflammatory Enbrel, a top seller, managed a 6.6% increase -- that beat Johnson & Johnson's (NYSE:JNJ) Remicade, which fell 2.4%, but it didn't come close to the 42% year-over-year growth that Abbott Labs' (NYSE:ABT) Humira managed. Next year, Amgen will have a larger marketing partner for Enbrel when Pfizer (NYSE:PFE) takes over Wyeth (NYSE:WYE), but we'll have to wait and see whether that's a positive or a negative thing for Amgen.

The bottom line looked better, but it takes a moment to figure out what's going on. On a GAAP basis, diluted earnings per share were $0.91, up 19.7%. Taking out various non-cash charges related to past acquisitions, adjusted EPS were $1.04, up 7.2%. Further taking out stock option expenses, adjusted-adjusted EPS were $1.06, up 6%. Amgen provided all three numbers. For the year, GAAP-diluted EPS was up 28.3% to $3.90, and "double-adjusted" EPS was up 6% to $4.55.

You know, it would be so much simpler if companies didn't try to show how things would be if you take out various charges mandated by generally accepted accounting principles.

Next year doesn't look quite as good. Earnings are expected to be between $4.04 to $4.31 (GAAP) or $4.55 to $4.75 (double-adjusted). That's not very much growth, no matter what you do to adjust the numbers.

While the company's revenue and earnings numbers are fun to analyze, Amgen's share price this year is likely to be determined by a drug that's not even on the market yet. Its osteoporosis treatment denosumab has blockbuster potential, as long as it can get by the Food and Drug Administration in October -- or perhaps earlier, if the company can get a priority review. It's also looking for an international marketing partner and is expecting data for using the drug to treat bone loss in cancer patients later this year. Both situations could give shares an added boost.

Amgen's anemic growth won't last forever, but investors who buy now are going to have to be patient.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer and Johnson & Johnson are Motley Fool Income Investor recommendations. Pfizer is also an Inside Value selection, and the Fool owns shares. The Fool's disclosure policy is worth a second read.