Adding custom spell checking to Word is easy. Office applications use lists of properly spelled words stored in simple Unicode text files will a file type of dic for dictionary. There is a legally free, GPL licensed medical list of words found at http://www.e-medtools.com/openmedspel.html. It isn’t in Unicode, so we’ve made a Unicode version of this custom dictionary file and packaged it in a zip file which you can download by scrolling to the bottom of this post. Under the terms of GPL licensing, you may download, modify, and redistribute this file only as a free product. GPL means once free, always free and this includes derivative works and any other kinds of enhancements to the original.
This is a simple process of downloading a zip file, extracting it to a specific location, and adding to Word. If you understand the process, it will take about one or two minutes. It is like many other IT tasks where reading the instructions takes more time than actually doing the task.
You can add custom dictionaries for engineering, foreign words, or whatever you need. Adding a custom dictionary affects all Microsoft Office applications, not just Word.
Windows 7, Internet Explorer 9 and Microsoft Office 2010 were used for the screen captures shown below. Microsoft’s official reference for adding custom dictionaries is found here.
Instructions for Word 2011 on a Mac operating system are found here.
Step 0. Open Word and find the location of your custom dictionaries.
Geek alert: If you are a geek, you can skip this step. The location you need is %appdata%\microsoft\uproof
Since I’m using Office 2010, I select File and then Options. If you have a different version of Word, see the Microsoft KB article.

Figure 1. Word 2010 Options.
Go to Proofing and click the Custom Dictionaries button.

Figure 2. Word 2010 Custom Dictionaries button.

Figure 3. Word 2010 Custom Dictionaries location shown to the right of File path.
Copy the location of your custom dictionaries. On my computer, it is C:\Users\John\AppData\Roaming\Microsoft\UProof as you can see in Figure 3.
Step 1. Download the zip file containing the custom dictionary.
Scroll to the bottom of this post and click the OpenMedSpel100.zip link.
Click Save as and save the zip file to your desktop.
Figure 4. Save zip file dialog box.

Figure 5. Save zip file to desktop.
Step 2. Extract the zip file to the Uproof folder.
Right-click the OpenMedSpel100 zip file and select Extract All.

Figure 6. Extract the zip file.
Specify the location of the Uproof folder found in Step 0. On my machine, this is C:\Users\John\AppData\Roaming\Microsoft\UProof.

Figure 7. Specify the location of the Uproof folder found in step 0 and click the Extract button.
Step 3. Open Word.
Go back to Word’s Custom Dictionaries dialog box shown in Figure 3. Click the Add button on the Custom Dictionaries dialog box.

Figure 8. Select the en_US_OpenMedSpel100 file and click the Open button.

Figure 9. Click OK and you’re done!
Step 4. Open Word or any other Office application to confirm your change.
Enjoy working with your Office applications without the frustration caused by spell checking false positives.

Figure 10. Microsoft Word before (left) and after (right) adding the custom dictionary.