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: Shares of drug developer Medivation (Nasdaq: MDVN) got a clean bill of health today, jumping to overnight gains as high as 13% on several times their average trading volume.

So what: The stock more than doubled yesterday as prostate cancer drug MDV3100 showed great trial results -- users of the drug tended to live about five months longer than those on placebo control duty. With any move that large, you should expect the stock to bounce again to adjust for overshooting or not quite reaching the full value of the fresh news.

Now what: Medivation is posing a large threat to Dendreon's (Nasdaq: DNDN) Provenge prostate cancer vaccine and the Exelixis (Nasdaq: EXEL) cabozanitib candidate now, assuming that MDV3100 passes all the FDA trials. Five months may not sound like much, but this drug is given to late-stage patients who already tried chemotherapy, where placebo-takers had just over a year left to live. And prostate cancer is a huge market: Intuitive Surgical (Nasdaq: ISRG) cut its robotic teeth on prostatectomies and now looks like the health-care stock to beat.

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Fool contributor Anders Bylund owns shares of Intuitive Surgical but holds no other position in any of the companies mentioned. The Motley Fool owns shares of Exelixis and Dendreon. Motley Fool newsletter services have recommended buying shares of Intuitive Surgical and Exelixis. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinion, but we all believe that considering a diverse range of insights makes us better investors. Check out Anders' holdings and bio, or follow him on Twitter and Google+. We have a disclosure policy.