The shares fell $14.47, or 31 percent, to $32.01 at 10:13 a.m. New York time on the Nasdaq Stock Market, after earlier dropping as low as $31.74 for the biggest decline since Sept. 29, 2005.

Advertising revenue in the second half of this year will be reduced by unexpected delays or cancellations of promotions from several consumer-product companies, New York-based WebMD said in a statement. Ads previously sold to drugmakers and consumer companies are going through extended legal and regulatory reviews, leading to longer-than-anticipated delays to introduce the ads on WebMD’s site.

The company said 2011 revenue will be $580 million to $600 million, down from a May projection of $610 million to $640 million. Income from continuing operations will be $71 million to $80 million, down from a previous forecast of $79.8 million to $91.8 million.