Spice up your Valentine's Day with even more smokin' hot investments.

Last year, investors fell in love with Schering-Plough (NYSE: SGP) and Merck (NYSE: MRK), which gave them hot returns of 14% and 32% gains, respectively. So far in 2008, however, the stocks haven't been on fire -- they've been in the fire.

Schering and Merck are still down more than 20% since the Enhance trial results suggested that Vytorin slows the formation of plaque in arteries as well as the generic, Zocor, does. While the press hounded the drug duo about the results, doctors were a little more mixed on whether the trial results meant that patients should be switching off Vytorin and its little sister, Zetia.

Of course, doctors are only one side of the drug prescribing puzzle -- all those direct-to-consumer advertisements on TV are proof that patients play an important role as well. The drug-takers seem to have bought all the bad press; several doctors have told me that many of their patients want to switch. Recent prescription data suggests that the rate of switching might be slowing, although it's too early to call a bottom.

Ultimately, what both doctors and patients really care about is whether the drugs help patients live longer. Large studies are still under way to address that question, and the drugmakers' futures depend on favorable results from them.

The interesting thing about the substantial drop in these stocks is that investors seem to be discounting the other drugs and the decent pipelines both companies have. Before the Enhance trial results came out, Merck thought its new products would be able to increase adjusted earnings per share this year -- even in the face of losing patent protection on Fosamax, a multibillion-dollar-per-year drug.

Schering has a well-stocked late-stage pipeline thanks to its acquisition of Akzo Nobel's (Nasdaq: AKZOY) Organon BioSciences subsidiary. Even with the Enhance trial results, neither of these companies is in as dire straits as Pfizer (NYSE: PFE).

I have no idea if we've reached the bottom of the slide for the duo, but I like their long-term potential. As the title of this series suggests, now is the time to start something with Merck and Schering.

If you do decide to fall in love and get married, realize the honeymoon will be short, and you may be in for a rocky relationship during your first year together.

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Fool contributor Brian Orelli, Ph.D., fell in love with biology in high school, with his wife in college, and with investing during graduate school. If he could get his wife to work at the Fool, he'd have all three loves in one place. He doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.