It didn't really matter what kind of numbers Merck (NYSE: MRK) released for the fourth quarter. The company and ally Schering-Plough (NYSE: SGP) are still reeling from the results of their Enhance trial, which showed a lack of plaque-lowering ability for their joint drug Vytorin. At least we know the peak from which sales of Vytorin and Zetia can fall -- $5.2 billion last year.

Merck is sticking with its 2008 guidance of $3.0 billion-$3.3 billion in equity income from affiliates. That primarily includes its cholesterol-drug venture with Schering-Plough, but also applies to partnerships with Sanofi-Aventis (NYSE: SNY) and AstraZeneca (NYSE: AZN). While I can't see how the partnerships will reach the levels Merck guided last month, it's a little too early to know how badly the trial results will hurt sales. Rather than take a guess that may need subsequent revision, Merck is keeping its mouth shut until more prescription data comes in.

While the cholesterol drugs were the focus of the earnings conference call, the rest of Merck's drug lineup is equally important. Merck is losing patent protection on Fosamax next month, jeopardizing the $3 billion the osteoporosis drug brought in last year.

Fortunately, its new drugs are performing well. In its first full year of sales, diabetes drug Januvia reached $668 million in worldwide sales, with sister product Janumet bringing in $86 million in sales for the year. HIV drug Isentress is off to a good start, although it needs to expand into a larger patient population before it can reach blockbuster status. Rounding out the stellar group of newcomers, Merck's human papilloma virus (HPV) vaccine, Gardasil, is benefiting from GlaxoSmithKline's (NYSE: GSK) FDA delay for its rival vaccine, Cervarix.

While investors may be having trouble riding the Enhance downward slide, there is one little advantage to this major drop in Merck's share price. The company still has $5.1 billion left to spend on its previously authorized buyback. At least at these prices, it can get more shares for its buck.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool prescribes a dose of disclosure.