Of course there are.

You don't have to install a separate medical spelling checker -- all
you need is to add a custom dictionary (really just a text file
containing a list of correct spellings) to Word. See
http://office.microsoft.com/en-us/assistance/HA010483191033.aspx.

A search on Google for
  "microsoft word" medical dictionary
will turn up thousands of hits for free, shareware, and commercial
dictionaries. The most commonly used commercial medical dictionary is
Stedman's (www.stedmans.com).

--
Regards,
Jay Freedman
Microsoft Word MVP        FAQ: http://word.mvps.org
Email cannot be acknowledged; please post all follow-ups to the
newsgroup so all may benefit.