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The Dow Jones Industrial Average (DJINDICES:^DJI) is pulling its weight for blue-chip investors today, picking up a modest 26 points as of 2:30 p.m. EDT to start this week off on a good note. Most stocks on the Dow are up, but don't thank Merck (NYSE:MRK) for the index's rise, as this big pharma giant has plunged by more than 2.5% after releasing its third-quarter earnings before the opening bell today.

It doesn't get any easier for Dow health care investors, as Merck's chief American rival, Pfizer (NYSE:PFE), steps into the spotlight tomorrow with its own third-quarter release. Let's catch up on what you need to know.

Merck's earnings take a tumble
Merck didn't light up any investors' eyes with its most recent quarterly results. The company's revenue dipped 4% year over year to miss analyst expectations, and Merck's profit plunged a far more troubling 35%, although the $0.92-per-share earnings figure did manage to top analyst expectations.

Merck tightened up its full-year financial guidance, but the biggest red flag of the company's release was in the details. Its top-selling diabetes drug, Januvia, once projected to be Merck's flagship product for this decade, has seen its growth stall out. Januvia sales fell 5% year over year in the quarter. When paired with sister drug Janumet, the duo lost 1% relative to 2012's third quarter.

What happened? Januvia and Janumet are seeing competition ramping up in the highly coveted diabetes market, and Januvia has also come under fire lately after researchers singled out the drug, along with a competitor, for an inconclusive risk of pancreatic cancer and pancreatitis in patients. Investors shouldn't worry too much about Januvia's safety profile -- it's a well-established drug that still had $2 billion in sales alone over the first six months of the year -- but they should raise an eyebrow over Merck's inability to keep this top seller growing.

With patent-cliff-related sales declines still hitting this company's revenue hard, it's critical for Merck that Januvia and other strong drugs with plenty of patent exclusivity left mitigate potential revenue pitfalls. That didn't happen this quarter, and Merck investors need to hope the company turns things around by the end of the year. Merck's stock had only advanced about 2% over the past 52 weeks before today.

Investors are hoping Pfizer won't fall to the same demons that hit Merck, but it's no question that the loss of patent exclusivity on former best-seller Lipitor will continue to hang over Pfizer's head this earnings season. Lipitor's sales have been slashed by more than half over the first half of the year, and Pfizer's been unable to compensate fully for the decline with growth from the likes of current top-selling drug Lyrica.

Wall Street expects a gain of more than 5% in earnings for the company, but analysts project Pfizer's revenue to continue dropping, with another 9% dive projected for this quarter. Pfizer's latest much-hyped drug, blood thinner Eliquis, has gotten off to a slow start, and according to partner Bristol-Myers Squibb's release earlier this earnings season, it hasn't made much progress, so investors should take a cautious approach to Pfizer's newest blockbuster hopeful before tomorrow's report.

Fool contributor Dan Carroll has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.