SteveWh - 3:40 am on Mar 10, 2011 (gmt 0)
Something in your site is testing whether the request is coming from googlebot, and serving alternate content if it is.
That can be done in .htaccess. Check your .htaccess for redirect code. Also search for new .htaccess files that aren't supposed to be there.
It can also be done by a malicious .php script embedded somewhere in your site. It would have to be in a script invoked for every page load; for example, a header or include file.
If your site is not just showing "links" but also the drug sales pages, it's possible the hackers have embedded their entire drug website inside your site, possibly bundled into a single big .php script with the contents base64_encoded to make it harder to find with text searches.