Without a medical spell checker installed, it is irritating to transcribe with MS Word with those red underscores all over the document. You get frustrated adding such underlined words every time to the inbuilt custom dictionary of Microsoft Word by right clicking and adding. To avoid such repeated annoyance, the more permanent solution would be installing a medical spell checker itself which works in the background.
A free custom medical spell checker for medical transcription in US English by a medical transcriptionist is now available! The word list of medical terms comes from the Open Source Medical Spelling Word List having around 50,000 words with terms updated until 2007 along with the words from my own personal collection and custom dictionary. Accommodating all these medical words, the number of words worked out close to 100,000 after filtering out duplicate entries.
For more information and installation instructions: