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 online health-information provider WebMD Health (Nasdaq: WBMD) were sent to the ICU by skittish investors today. The stock price fell as much as 32% on huge volume.

So what: WebMD "updated" its full-year 2011 guidance today, which, as you might guess from the market's reaction, meant that it cut its projections. For the year, the company now expects revenue in a range of $580 million to $600 million versus previous guidance of $610 million to $640 million. Income from continuing operations is seen clocking in at $71 million to $80 million, down from a previous estimate of $80 million to $92 million.

Now what: While the guidance cut may not seem that drastic, investors may be reacting to the reason for the paring. WebMD said that the lower-than-expected results were driven by sponsorships that had been sold and are now being cancelled, as well as launch delays in other programs due to increased internal legal reviews. With the vast majority of WebMD's business coming from advertising and sponsorships, it's not all that surprising that investors would freak out over a threat to this business line.

Bloomberg noted that this is the largest drop for WebMD's stock ever. Does that mean investors could be overreacting? It's certainly possible, but investors have been willing to pay a very healthy valuation for this stock, so it doesn't shock me that growth concerns would send shares swooning to this extent.

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Fool contributor Matt Koppenheffer does not have a financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or Facebook. The Fool’s disclosure policy prefers dividends over a sharp stick in the eye.