Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

Stocks are nosing higher to end the week, with the Dow Jones Industrial Average (DJINDICES:^DJI) pushing toward the 16,000-point mark. It's only about 60 points away as I write, having gained 65 points as of 2:35 p.m. EST. Stocks are having a slow day across the Dow Jones, but most Dow components are in the green, with ExxonMobil gaining 1.8% to lead the index higher.

Health care's having a sluggish day, with both Merck (NYSE:MRK) and Pfizer (NYSE:PFE) hanging flat so far. Both of these pharma giants rely on their best-selling drugs to keep their cash flow up -- but which one has the better portfolio of blockbusters? Let's compare some of their top products.

Merck rallying behind Januvia
Merck's diabetes-fighting duo of Januvia and Janumet exploded onto the scene and have soared to become the company's top drugs, with sales of more than $4 billion combined through the first nine months of the year. However, revenue from these two drugs has actually declined on a year-to-year basis despite the diabetes market's surge and the growing worldwide prevalence of this disease. Considering what the patent cliff has done to sales of former top sellers like Singulair, Merck needs Januvia and Janumet to return to growth.

They still have plenty of time, because neither drug goes off patent for quite some time. The same can't be said for top-selling cardiovascular drugs Vytorin and Zetia, which will each lose patent protection in the coming five years. While Vytorin has not been much of a growth candidate lately -- it lost more than $100 million in sales over the first nine months of 2013 on a year-to-year basis -- the two drugs combined made more than $3 billion over the first three quarters of 2013. That patent loss could hurt.

Pfizer is going strong with Enbrel and Lyrica. The company only sells Enbrel outside of the U.S. and Canada, but this drug is among the world's top-selling therapies, having delivered more than $2.7 billion in sales over the first nine months of the year. Growth, though, has been elusive this year.

Enbrel won't suffer patent expiration for a while, but Lyrica, Pfizer's top seller that made more than $3.3 billion over the first nine months of the year and grew revenue by 10% year over year during that time, will lose U.S. exclusivity in 2018. That's plenty of time for Pfizer to capitalize on Lyrica's strong growth -- especially strong for such a potent blockbuster -- but Pfizer investors do need to look beyond Lyrica to up-and-comers like Eliquis.

Overall, however, both Pfizer and Merck's best-sellers still look like good bets going forward. It's what the companies' pipelines can produce, though, that will really impact these stocks' long-term futures once patents begin to expire.

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.