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

What: Despite beating Wall Street's estimates for first-quarter results, shares of Elan (NYSE: ELN) plunged as much as 15% in early trading today. The stock has since recovered to be down about 4%.

So what: Elan earned $0.12 per diluted share on $313 million in Q1. Analysts had been calling for a $0.03 loss on $297.04 million, according to Yahoo! Finance data.

Now what: It didn't matter. Biogen Idec (Nasdaq: BIIB), an Elan partner in marketing the MS drug Tysabri, reported promising trial results for a competing MS treatment called BG-12. (Click here to read about Biogen's stock pop.) If approved, the new drug could cut into Elan's Tysabri sales because it's administered orally rather than intravenously. Tysabri patients are also regularly screened for a rare brain disease called PML, which can manifest as a side effect.

Investors' fears of Tysabri, which accounted for 78% of Elan's first-quarter revenue, being replaced may be justified someday. For now, it's important to remember that Elan easily beat estimates while Biogen's BG-12 remains in a phase 3 trial. Mr. Market is likely overreacting. 

Interested in more info on Elan? Add it to your watchlist.

Interested in more info on Biogen Idec? Add it to your watchlist.

Fool contributor Tim Beyers is a member of the Motley Fool Rule Breakers stock-picking team. Elan is a Rule Breakers recommendation. You can try any of our Foolish newsletter services free for 30 days.

Tim didn't own shares in any of the companies mentioned in this article at the time of publication. Check out his portfolio holdings and Foolish writings, or connect with him on Twitter as @milehighfool. You can also get his insights delivered directly to your RSS reader. 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 is also on Twitter as @TheMotleyFool. Its disclosure policy is at least 10% better than other disclosure policies.