Hosford Medical Terms Dictionary v3.0 is offered as a free text file that you can import into Word as a custom dictionary. (A voluntary donation of $3 is requested, and is probably worth it if you use medical terms regularly.)

First, download the zip file at: http://www.ptcentral.com/university/medterms_zip.html. Then, extract the files and save them in a directory where you can leave them permanently. Next, start Word and do the import:

Select Tools, Options from the menu bar.
Click the Spelling & Grammar tab.
Click the Custom Dictionaries button.
The “Custom Dictionaries” dialogue box opens.
Click New, then browse to the folder you saved the dictionary files in.
Select “MedTerms.dic,” and click OK three times.

Hosford Medical Terms Dictionary v3.0